Glad I could point you in the right direction.
Consider asking for advice on the jenkinsci-dev mailing list. I'm pretty sure it's not too much effort to do a kind of "import configuration" step (basically during XML deserialization, readResolve and all that) so that your rewritten code could work with existing configuration. You can also make it a semantically versioned "2.0" and note some incompatibility to previous versions in plugin manager/update center to make users aware of the big step.
I think there's value in continuing work (at least from a migration of on-disk configuration point of view), as having multiple plugins around that do essentially the same thing (assuming that's the case) is a weird situation for everyone. Plus, you'll inherit the users of the existing plugin, finding bugs for you Just my suggestion, how you decide to proceed is of course your choice.