Can I upgrade the processor in a 2018 MacBook Pro 15-inch after buying it?

Regarding the MacBook Pro: No, the Core i7 or i9 is soldered to the logic board, so whatever you picked at checkout is what you’re stuck with. This is the era where Apple committed to the unified, unupgradeable motherboard layout on the Pro line. If you think you might need the i9 down the line, that decision is made on day one.