Re: Replacing square recessed lights
My suggestion is to install tounge-and-groove boards of the same size & style perpendicular to and just below the existing boards, and place some kind of trim around it. Kind of like a 12" square medallion; it would project about 3/4" from the ceiling and the round can light would be centered in it. It could be painted or stained to match the existing ceiling.
By placing it perpendicular (90 degrees) to the existing, I think it would look better and be a whole lot easier to do than trying to match & patch the existing stuff exactly.
Your square lights probably have a depth above the ceiling of about 3-1/2 inches. The round recessed lights probably have a depth of 10 or 12 inches. Make sure you have enough vertical clearance, and if there is or ever will be insulation in the ceiling, that you use an IC-rated fixture.
(Man, this would be easier if I had the skill to draw a picture.)
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.