Install is creating all the correct (but unwanted) unconfigured BASServer.props (BASServerMySql, BASServerMSSQL etc) files with each installation over a prior install, but it appears to be adding a BASServer.old which appears to be the configured BASServer.props (ie it contains all my DB connection strings).
It's definitely install related as I never replace file suffixes with .old, I put it into the file name.
Just wondered if the install process was making it a bit less of a pain to recover prior config data when updating.