Most likely they will have pre-assigned you a car before you get there, which should include the upgrade. If not, they will assign one when you are there. In both cases you can ask for a specific type/model of car as long as it's in the class you booked (or one higher, given the upgrade) - but at a smaller location the simple fact is that the range will probably be fairly limited and you'll be lucky to have much of a choice.

If they've already assigned you something they changing shouldn't be a problem - again subject to what they have available (which will NOT be the same as what's in the lot, as some of those will have already been assigned to others).

If you're asking for something higher than you've booked (taking the upgrade into account) then they will likely be happy to comply, but there will be a fee, and it'll probably be more than just booking the higher class in the first place.