Optimum, safest way to migrate one user to another - 20.04

20.04, effectively a kubuntu system.

User 'adam' (/home/adam) uses a KDE Plasma desktop.
User 'glen' (/home/glen) uses a Gnome MATE desktop.

I need to migrate daily use of 'adam' (settings, program data, etc.) to 'glen'. I think data stored outside /home/xxxx will require considerable "chown & chgrp" operations. For example, the files & subdirs on an external HD/partition '/extra' is entirely assigned to user 'adam' with owner 'adam'.

I'm not making 'adam' with its /home/adam go away. But I want to change which user is utilized daily, which will eventually be 'glen'.

Is there a guided or safe, comprehensive way to accomplish this? Thanks.