I have just changed handsets so here is my experience.
Put a SIM in your old handset. It will not work without a SIM card installed.
Enter your code to get into the avast menu.
Disable Anti-theft.
Exit.
Go to your phone Application manager.
Find the Anti-theft and choose uninstall program.
Anti-theft will be removed from your handset.
You can do a factory reset after this (I did) to make sure old phone has no remaining issues or personal details.
In your my.avast account choose the handset you are not using anymore and delete it.