Update: Even no exceptions was throw. The add-on still enable after upgrade. (I have disabled before upgrading). It's on site nobita.me with XF 2.0.6 version.
I recently updated an add-on by deleting first the old version of it.
On the server I deleted the add-on files and replaced these with the new files.
Back in Admin I first renewed the page and thereafter I could install the new add-on.
Update: Even no exceptions was throw. The add-on still enable after upgrade. (I have disabled before upgrading). It's on site nobita.me with XF 2.0.6 version.
I recall now that this is deliberate. I can't recall the exact reason. Likely it's to avoid problems arising from things such as code events and listeners being unavailable when templates are being rebuilt and that kind of thing.