Every single time Illustrator updates to the next version and deletes the previous one, I LOSE the Roland Cut Studio plug in. EVERY SINGLE TIME. Every time, I eventually find it and get it reinstalled. This time is different. I can't find it. I went to an old version and moved it over to "extensions" under the new version of illustrator, but it is STILL not showing up in the program.
Based on a quick search on Roland's site, the Plug In is not yet compatible with Illustrator v26, as there are many people having the same issue.... and that is Roland's issue to fix.
As far as the initial issue, each version has its own Plug In folder... your plug in is not automatically moved forward, but even if you had Adobe delete the previous version when you updated, that Plug In folder by itself should have stayed behind so you could move it from the old to the new.