Real Dictionary

trade up
phrasal verb: If someone trades up, they sell something such as their car or their house and buy a more expensive one.

trade up in American English
to exchange a less valuable or desirable item for a more valuable or desirable one
to trade something for something of higher value

trade up in British English
verb: to sell a small or relatively inexpensive house, car, etc, and replace it with a larger or more expensive one