I'm an avast user like yourself not an avast team member, but I would suggest that you keep copies of your modified files.
Avast has an integrity checker function, when and why that might run (avast program update/repair, etc.) or to what files it applies I can't say. But if it did check these .txt, .htm files there is a possibility they might be replaced.
As you mentioned program updates, a clean install would certainly be likely to replace. I don't know for sure if updates in place (UI update) would replace them, your settings and modules should remain the same.
In earlier avast versions I modified the .txt and .htm files without issue, including in place program updates, but always kept copies, just in case. How this might fit in with the latest version, only time will tell.