hi, welcome... I'm afraid you've taken the most difficult possible route here... Phantom 2 VISION series were designed to run with the Vision gimbal/camera only, it's not exactly easy to "convert" it to a separate gimbal/gopro.
You're correct that your remote control operates on 5.8Ghz so therefore you shouldn't put in 5.8Ghz FPV... you could pretty easily put in 2.4Ghz FPV transmitter, but I'm not sure any 2.4Ghz receiver/displays to recommend.
you ask "is it possible to change the transmitter on the quad" ... for remote control, the transmitter is actually in the RC and the quad is the receiver... so if you want to convert the quad to "operate" on 2.4Ghz (like the non-Vision series Phantom 2) you're going to be opening up the quad, completely disassembling it, replacing receiver boards and other sensitive equipment, and you'll have to buy the 2.4Ghz version of the remote control (the DJ6). That task is MUCH easier said than done!
Are you sensing yet that you're going to be spending a lot of time and money doing this conversion?
I'm not sure the exact numbers but I suspect they're going to offset any cost savings you got buying a used P2-Vision with no gimbal ...
I would crunch some math you may be better off just selling a working P2V with no gimbal (assuming you test it properly so you know it's working) and then go get what you ACTUALLY want/need which is a P2 non-Vision series on which you can easily add a gimbal and 5.8Ghz FPV.
or just get a Vision gimbal/camera but it sounds like you prefer the gopro which is indeed way better