The 3/4 inside of a 1" for 20 feet will drop the pressure only
depending on your flow rate. You can do this yourself..
If you do, try this:
After you have cut the existing 1 inch pipe at both ends and
fed the 3/4 pipe thru it, see if you can rotate the 1 inch pipe.
If you can, and it is loose, then you might be able to remove
it and put a new 1 inch pipe thru the hole, using the 3/4 inch
pipe as a guide. Sort of like running one piece of pipe along
the outside of another. If it works, you can then take the 3/4
back out and just bond the 1 inch in...
I've never done this, but , IF the 1 inch pipe is in the
right kind of soil, this would be worth trying. At the worst,
you still have the 3/4 pipe in place, as before.....
If you try this, and it works, please report back here.
I'm sure we'd all like to see if it works....
Andy in Eureka, Texas