As previously noted, old galvanized is a scenario that implies
replacing the line, but better now in the decent weather than in the
middle of a blizzard or somesuch!
As another noted, a "trench" can be a lot less intrusive than a
full-fledged backhoe these days and there are techniques that can be
used if you're in a location of sufficient population to have somebody
in the area that has the equipment. If you're calling in the pros
anyway, it's only money!!! :)
I'm thinking actually it won't be as bad as you may be dreading
assuming you can find somebody who is pretty good to do it...again, as
others have noted, unless there are other obstructions (like the drain
field, rock, etc.) or you're simply out of room on your property it
shouldn't be difficult to avoid doing major damage to the tree.