Yeah, you're right. You don't need to worry about that if you're upgrading to a bigger memory card, as you can have as many memory cards as you want tied to the same username, but you'll have to restore the Vita to factory settings each time you want to use a memory card with a different username tied to it.
Edit: Sorry, I think I may have misunderstood you earlier. You're correct that your PSN username is tied to the memory card, so you'll have to format it if you want another username on it.