Usually within NYC (Manhattan) when needed to swap out something (low fuel, damaged, only thing left but not what was reserved, etc...) counter reps make phone calls and clear the way so to speak. That being said they most always ask if the location has inventory to spare. If they don't then things become interesting. Sometimes they will others it takes a manager getting involved.

Regardless of airport or other location places only have so much inventory and they must keep that supply balanced with reservations.

Don't know what you mean by a "captive market", but airport locations have reservations same as those in say Manhattan. If I reserved a mid-sized car then that is what I probably wanted. Have been stuck with SUVs many times from National because of this or that inventory problems.

You had your chance to speak up when you took the RAV4. If you didn't like it then the time to say something was before you left the location and or shortly afterwards. Not driving out of the City then deciding you want to make a swap.

There is also the situation where certain vehicles within National's system are only at airports or urban locations, that or vice versa. I've seen some nice vehicles sitting at local National locations in Manhattan only to be told it couldn't be rented because it belong elsewhere and was waiting for a hiker to take it back where it belonged.

This last bit could be another issue; logistically the vehicle (your RAV4 for instance) "belongs" in Manhattan at the location you rented it form and not upstate. Unless this was a one way rental the location you took the vehicle from is expecting it to return when you said it would. If or when it does not then the location in question may have a problem. What about the person who reserved a SUV and National is counting on your rental to make that quota? Leaving all this aside there is the question of how the Albany location (or whatever outside of the City) is going to get the thing back down here. If they must use a hiker that incurs a cost which was not necessary because if you had done what the contract called for it would be returned on your dime.

If the RAV4 was not functioning properly or something major was wrong don't think many locations would refuse a swap. But for you to just swan in there saying "I don't like it" is another matter.