Installing a new recessed light housing at locations I already have old housings

OK I have a tricky situation.
I have to install quite a few recessed lights.
Most of them I have accessed (ceiling down), some of them I do not. Of the ones I do not, some of them I need to drill large holes to mount them, and some of them have existing non-functioning recessed lights already in place I need to remove first.
I intend to use as much as I can get away with IC housings for new work (not remodel) which means I need to get adequate access to the space above.
The ones I already have 35 year old beat up non-functioning rusted housing up there, with the flanges attached to the joists, and I need to attach a new one (different diameter) I will have to figure out a way to remove them, and attach new ones. To make the matter more complicated, the ceiling is not regular sheetrock, but a 3/4" thick material which is composed of 3/8" of plaster on top of 3/8" of gypsum board backing so cutting must involve a carbide blade.
I do have access to one of these:
http://www.holepro.com/powerseries.html#X-det
and can cut a large hole up to 9" in diameter. I think my best bet may be (I already disconnected the wirings) use a hammer to knock the existing can back into the attic, then insert a piece of 1x2 wood into the hole, and secure the wood to the drywall across the middle from the inside using a few screws, then use this shielded hole cutter with the starter screw going into the piece of wood in the center and cut a 9" big hole. I tested this with a piece of scrap wall I demolished, and it cut through it like butter and no dust.
That would allow me enough room to cut out the existing housing and flanges, and probably enough room (I hope) to mount a new housing to the joists. The advantage is I can easily cut an identical piece of 9" hole and the patch would be perfect, and if I do that for all the holes I do not have access (about 16 of them), the patching work would be the same and no need to waste time measuring each hole being cut slightly different sizes etc...
One other upside is that this hole cutter has a plastic see-thru shield that collects the dust. Using a rotozip angled up even wearing eyegear there is no way to stop the nasty plaster dust from getting into your eyes and nose.
The downside is I would be patching a round hole, which I have not done before, how would you apply straight drywall tape to a round hole? Do you use short strips?
The other downside is in the event a 9" hole is not enough for me to mount the new housing, I would have to further enlarge the hole until I can...there is no going back to using remodel housing since the drywall support from a patch would be inadequate.
Any thoughts or comments or alternatives?
Thanks,
MC
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*If you are able to get into the space above to install new recessed light housings then why don't you just remove the old ones at the same time. They are probably just nailed to the joists.
To remove from below you should remove the trim and bulb. Then loosen the interior screws or clips holding the can in place. Some are just held up with friction and can be pulled with a good tug. Cut out the can and you are left with the frame in place. You should be able to reach up through the hole with a hammer or pry bar and pull the frame brackets away from the joists. I doubt that you will be able to pull the frame down through the existing hole.
I have never used that particular tool for cutting holes. I used a cheaper version from Greenlee once and tossed it. I have also used the specific hole saws for each diameter recessed light with good results however they can create a lot of dust. Most of the time I just use a carbide tooth compass saw and cut by hand and keep a small box on top of the ladder to catch the bulk of the dust. I am always afraid of cutting into wires and pipes with the power tools.
A 9" hole is quite big for recessed lights. Are you installing commercial fixtures?
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Thanks, the attic above is real tight I cannot get to it from above for most of the lights. I have to deal with it from below.
I am not installing 9" housings. I am installing 5" housings. I am saying to use the shielded hole cutter to cut a large hole so I can have access to remove old housing and mount new ones (not the remodel kind).
The problem is even if I can knock out the old can the existing holes are too big, the existing holes are 6-3/4" for the old housing and this is too big for the new housing anyways. I doubt I can mount a smaller remodel can onto a larger hole. So my thinking is to make the biggest hole possible, mount the new lights to the joists through the 9" hole (if possible, but I am really uneasy about this, I think I might end up having to cut a 16"x16" square hole).
MC
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Thanks, the attic above is real tight I cannot get to it from above for most of the lights. I have to deal with it from below.
I am not installing 9" housings. I am installing 5" housings. I am saying to use the shielded hole cutter to cut a large hole so I can have access to remove old housing and mount new ones (not the remodel kind).
The problem is even if I can knock out the old can the existing holes are too big, the existing holes are 6-3/4" for the old housing and this is too big for the new housing anyways. I doubt I can mount a smaller remodel can onto a larger hole. So my thinking is to make the biggest hole possible, mount the new lights to the joists through the 9" hole (if possible, but I am really uneasy about this, I think I might end up having to cut a 16"x16" square hole).
MC
In your situation, I'd cut a rectangle out, beam to beam, then you can easily remove the existing frame, and install a new, new work frame. It'll be easier to patch in a nice clean rectangle than a small circle. You also won't be able to attach a new work frame through a 9" circle
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I just did this for my bathroom. I had 2 cans that were non IC, non AT that every winter just got a layer of ice on them from the cold air pouring through from the attic. My ceiling is just as you described, 3/8 gypsum w/ 1/2 inch plaster.
I cut a giant square hole around the fixtures, back to the studs, to give me access to the old fixtures and then just patched it with a couple of layers of drywall. Using various thicknesses of draywall I was able to match the thickness of the original ceiling. It just doesnt make sense to work in a tiny hole wiring things up and trying to make the new fixture fit. It was just so much easier to open the ceiling and do the work. If you have to patch a ceiling regardless, it makes no difference whether the hole is large or small.
Good thing I did that too. The old fixtures were framed in with 2x4s to hold them in place. That all had to come out to make room for the new fixtures.
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John, that's so funny about the hole cutters. I bought the Greenlee, used it once, and tossed it. What a piece of junk. I use the blue, carbide tipped hole saws with a half of a basket ball slipped onto the hole saw to catch the dust. I only use them on plaster ceilings, and generally only cut through about 3/4 of the way, then finish it off by hand with a compass saw. If that 6" or 8" hole saw binds on you in the middle of the cut, it's tough to hold on to the drill, and or the ladder.

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*Roy that half of a basketball sounds like a good tip. You should send it into Electrical Contractor Magazine and maybe it will get published. I sent in a tip several years ago and won first prize; a couple of hand tools and a fishtape from Klein. Fortunately I don't see plaster ceilings that often anymore. Mostly condos and townhomes with high selling prices built by low bid contractors to minimum standards.
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