More specifically, is this acceptable at a smaller non-aisle airport location?

I will be renting this weekend and next week for leisure and currently have the following reservations:
Sat - Tues -- Premium
Tues - Weds -- Premium
Thurs - Fri -- Premium
Total cost: $160.00 / 5 days
2 credits

What I like about the above reservations is that a) premium will essentially allow me to have my choice of the lot, and b) I will have multiple options to switch things up OR keep the vehicle I'm in.

What I'm considering is --
Sat - Sat -- Fullsize
Total cost: $165.00 / 7 days
1 credit

So, I'll have two extra days and no requirement to return to the airport to swap out cars if I don't want to. However, coming in on a fullsize reservation and asking for a Suburban or Convertible is *likely* to be fulfilled, but I would feel less confident about it than having a premium reservation.

However, if I did go with a 7 day rental and decided I wanted to switch things up midway through the rental, is it acceptable to do so? Is this common or will I just annoy them with this request?

Thanks!