Yes they are interchangeable. Convertible back seats are different and they aren't interchangeable.
I think I've had 14 bmw's in 4 years.
2 IX's -sold and parts
2 325i 4 dr - sold
4 318is all white - sold and parts
1 87 325is red - parts
1 89 325i vert grey - parts
1 90 325is white - keepin
1 87 325i vert -red - keepin
latest addition 1 88-325is red - parts
latest addition 1 89-325i vert - white -fixin/sold
latest addition 1 92-318i vert - blue - parts
latest addition 1 87 325is blk - parts.
latest addition as of 2013-Gold 87-325is auto! it's a keeper!
For the rears the only difference is the lower half, coupes are a little more squared off, whereas the sedans are more rounded on the edge so passengers can get in. You probably won't noticed a difference.
Also, let it be noted for future references, that coupe front seats fit into convertibles, but the rear passenger sliding handle (that flips the seat forward) needs to be modified if you are tall. That handle sits higher on the couples, and the metal inside the handle needs to be cut down and ground down so that the cabrio handle will slide over it.