Does this statement mean that this plugin will cease to exist or that it will switch to a paid model?
Soap Skin & Bubble has always been free but the author put in a time limit which requires the user to update/reinstall it once in a while. TBH, though, I don’t remember having had to do that for a long time. Maybe they removed that requirement.
pretty sure migrating your SU version counts here. so, same. but I also just got the message, I’ll reinstall it when I need to.
and that’s odd, right ? 
I wouldn’t expect migrating to renew the license since SketchUp’s migrator only copies them from the old version to the new, right?
Yes. It is odd. I wondered about it when it was originally released. Just assumed the author expected to do make it a paid license at some point but never did.