Ok I’m super smart
Here’s what you do. You take this J section out. Cut out a section. Let’s aim for 8”. So mark and cut out 8” piece
Then measure that piece. Whoops its 8 1/4. Or maybe its perfect and 8. Whatever
Get a new piece of identical pvc and make a piece 1/2 longer than the one you removed. Then use two 1” - 1” couplers, glued on each end of the new improved piece. and reassemble into the J shape
Picture left as current with the marked section being your cuts. And right is reassembled with a new middle that’s cut 1/2” longer
4 couplings and 1’ of pvc should do it if you cut out 5” and make your new sections 5.5”