You can abandon your plot with /plotme dispose. All your builds will stay on it for anyone to claim and use. You can also clear your plot with /plotme clear, this will reset your plot to square 1. If you find an empty plot you like, you can do /plotme claim while you're on it (if you've disposed of your other plot), or you can just do /plotme auto. (Keep in mind that it will give you the first available plot, which might be the one you just unclaimed.)