Originally Posted by Ldnn1

Err, those are very broad questions. You'll need to be more specific with e.g. what rental companies you're considering, when, what car you're booking and what you'd hope to upgrade to.

Car rental companies everywhere upgrade all the time, most frequently because they don't have the car you booked left. Some also upgrade if you have status in their programme, but you don't mention that so I'm assuming you don't have any.

Without further information, you won't be able to get a helpful answer.