Gentoo's official XFce guide (now migrated into wiki view) warns:

«If you're upgrading Xfce from earlier major versions (4.x), then you will need to remove your old cached sessions and profiles as they are incompatible with new releases. For each of your users, run the following commands to remove your old incompatible cached sessions and profile:
# rm -r ~/.cache/sessions
# rm -r ~/.config/xfce*
# rm -r ~/.config/Thunar

Users will be greeted with a new and shiny interface, but will lose many of their individual settings. Sadly, no migration of configuration(s) exist that we know of.»
https://wiki.gentoo.org/wiki/Xfce/HOWTO#Upgrading_Xfce

To my experience, such action sometimes is necessary not only after major update, but also after minor update (last time from 4.10.0 to 4.10.1, GUI poweroff function was broken, AFAIR with conftest core dump on session's save attemp).
The necessiry in complete restore use's settings is very unpleasant.

So, utility to check existing profile compliance with currently installed xfce utilities is wanted.
With function of display/interactive clean (up to auto-skip incompatible or broken ones) user's session files.
Ideally on final point — auto or on demand (according user's preferencies) convertation user's profile data files from old format with check/warn about broken ones.