I feel like I'm being rude, I'm sorryyyyy D: But it's still owned by my "old name." I can still build on it and everything, but I can't do things only the owner could do. (ex: /deny, /add, /plotme clear) Oh and I tried, but it said that it's not mine again.
I'm pretty sure changing your name and trying to delete your plot is similar to getting a new Minecraft account and trying to delete your plot... Either way, /plotme dispose isn't working.
My friend changed his name, he gave the new account permission just in case something goes wrong. He has access to the plot but not the owner commands. You'll probably never have access to your plot commands after the name change.