Welcome 'lurker'
Installing Pro8 should retain any of your Free8 personalized additions to Plugins etc.
However, if you were to go 'cross-version' then v7 stuff stays in the '7' path and you'd need to copy personal stuff over from any v7 folders into the equivalent v8 folders...
Once you have finished your 8Pro trial you can choose to license it and what you had will be retained.
Alternatively, if you revert to 8Free you will need to 'uninstall' 8Pro: however, once again any customized files in that installation's folders should be kept and then re-recognized by the new Free8 installation that reuses the existing folders...