I have been doing this for years - if it's an off-airport branch where you need go inside and talk to someone, the chances of keeping the same car outside your reserved car class is very limited especially if there is a significant gap between what you are driving and what you reserved with that branch. In fact, I had a horrible experience for a long time at the Anaheim branch with one pain in the a** agent who got personal kicks and giggles from trying to take a nicer car from me within my reserved car class and give me a smaller worse piece of crap off the lot, and his behavior ended up being escalated to senior regional management.

If you really want to keep the same car, you need to do it at an airport location and know the layout of the location - for example, at some locations like LAX, you return downstairs and rent/check-out upstairs and you cannot drive the car yourself back up and through the exit, and other locations may not have exit booths you drive out from (LGB, for example) where you need to speak to an agent anyway. Know the layout of the location to make sure you can drive in, close your contract, and then be able to access the exit booth easily without the return lane agents making a fuss.