A: Wow! You bought a new car.

B: Since my wife gave birth to our son, I’ve been planning to trade up from my little car to a much larger one.

A: I can’t afford what I have, let alone be able to trade up to anything.

Note: This idiom means “to trade something in for something more expensive or valuable of its kind.” In other words, it’s an upgrade, signifying that the trajectory is upward instead of downward. The opposite is “to trade down.” For instance: They were forced to trade down their house after Jim lost his job and they could no longer afford the mortgage.