I found when I was testing making apartments I couldn't change residential lot types in Bridgeport. They were locked, but am not sure why. It's the same with University as well.
What town are you building in?
I tried a building cheat (EnableLotLocking on/off) but that didn't seem to help the residential lot types to become available in either Bridgeport or University.
I'm adding a link to Carl's Sims 3 Cheat list anyway, just in case you want to do more experimenting.

Carl's Sims 3 Cheat List:
https://www.carls-sims-3-guide.com/cheats.phpMy apartments that I've made were built in Sunset Valley and are regular residential lots. It was bugging me that when using the Apartment type lot the game would automatically add roommates to the lot and you can't turn that off by default. You need a mod to do that with. So instead of having to add yet another mod to my mods folder I chose to build the apartments on regular residential lots.
It ends up not being a rent price, but a smaller purchase price. I'm okay with that.
Thank you for reminding me of that issue. I'm going to add it to my How To Create Apartment notes,
here. Under More Tips and FAQs - Can't Change Lot Type.