Claim your work with ./kit protect. The base must be 10x10 or larger, and right click the shovel on both corners (diagonal) from each other. /trust [name] to trust people to your work. But only add people who you ACTUALLY trust.

Yes, once you change your username you do not have permissions on your plot if it's under your old name.

Ah, that sucks. So I'd have to change it back to get permissions?

Yep