OP - As this is a change in operating carrier, you are entitled to a full refund to your original form of payment and can then purchase whatever you want. As a general matter, UA will likely acommodate a fee-free change, but it could simply refund your ticket if it wishes.

I do think that the term "regional jet" is a bit silly in this day and age. There are CRJ-200's and there are '175's. Everything is a matter of personal preference, but to be frank, the '175 has 1-2 seating in F and 2-2 seating in Y, the pitch is roughly the same and is at least as comfortable as the standard narrow-body mainline aircraft.

F food is being upgraded to a meal, although the lack of a heat source will mean cold meals only.

Of all the reasons to select a carrier or flight, I would not make the distinction on operating carrier, but rather on hard/soft product and general classifications such as RJ won't get you that.