The Gen 1 rotors are roughly the same OD as the solid ones, however they use a larger pad, yielding more swept area per ton as compared to the solid rotor pads. They also look a little beefier, even though they're the same diameter.
You don't need new knuckles to swap them over. You need:
89-92 vented rear disc parts:
-Backing plates
-Caliper brackets
-Calipers
-Pads
-Rotors
Ebrake cables and hoses are the same, so unless yours are shot you can re-use your old ones.
The swap may or may not be worth it, depending on why you want to swap. The backing plates are a little hard to find new, so getting them at a junkyard, or from a parts car will most likely be your best bet. May as well buy the used calipers too, since rebuilt calipers come with a core charge. Different stores vary on their core charges, but I've seen them as high as an $80 core charge for a Gen 1 caliper. The $10 or so you spend on a used one will end up saving you money. Caliper brackets and calipers are available new or rebuilt almost everywhere.
Also, the vented rear setup will also allow you to upgrade to the 11.6 rear brake setup in the future, if you desire to do that.
