Yeah, it depends on what you want. You might not need to upgrade anything, it's all up to you.

On my Luna, I will eventually upgrade my pedals. But that's it. I already replaced the double crank with a compact double.

On my Bike Friday, the only thing I want to get is a rearview mirror, because it's a pain to switch back and forth with the Luna. That's not really an upgrade.

I will invest in a major maintenance overhaul on both bikes, though. Every few years it's a good thing to do, if you want to keep the bike, that is.