Well after making a note of my settings etc, I soft uninstalled ASB via Control Panel, choosing to retain browser data instead of total cleanout.
Then downloaded setup file from Avast and installed. I did a custom install just to omit icon from taskbar and omit starting the browser straight after install.
All seems well. I now have ASB Version 77.2.2154.121 (Official Build) (64-bit) instead of 32bit, whilst retaining my settings, extensions, bookmarks etc.