Rated 5 out of 5 stars

That sounds good to me.
Will the old (changed) subject lines be adopted when switching to the new version 2.3?
I'm currently still using Thunderbird 60.x with the old version 2.1.1 and I want to avoid losing all changes when upgrading.

I've tested it (with option "local only"): There's no data loss during the upgrade. The old changes in the subject lines are kept.

Léirmheas ar leagan níos sine den bhreiseán seo (2.3). 

If you unchecked the local only option it won't be a problem.
If you did make your changes local only I'm not sure. This is a different extension and I haven't tested this.