[wp-hackers] Can't rely on activation hook running on upgradation?

[Raj Sekharan]

I recently made a release to a plugin in WP.Org. The newer version has a
different database structure than the previous one. Some critical functions
depend on it. The table is altered after creating them in the activation
hook. However, after an upgrade many of my users are reporting not being
able to use some functions because of the databse structure issue.

Not sure if this is a bug or I'm missing something. What needs to be done
to ensure that some code is always run in initial activation or upgradation?