I don't usually intend to upgrade but if a better one happens to come into my possession - in a lot, through the thrift store or a a good price, I usually have no problems selling the old one off. This way, I have more money for ponies and my old one gets a new one where (s)he gets to be loved and appreciated.
My only exception to this is if the pony has special memories attached - like a gift from a loved one or a childhood pony. Then I may keep both.