The only consent I ever give avast is permission to run the full offline installer (after notification), and even then I always use the Custom Install feature,
never the gui upgrade.
I never have issues when done this way.
I always do a full uninstall using Programs and Features along with avastclear.exe in safe mode. Certainly helps to prevent unwanted surprises 100% of the time.
I'm certainly not holding my breath to see if avast will fix the issue on an upgrade path. I'd have died a long time ago waiting.

Having a copy of Unchecky running in your systray has helped in the past when the unexpected happened during an avast full install, just so you know. Has blocked two times so far.