I've never enabled/disabled junk mail handling for our GW clients. I'm
wondering what will happen if I lock the setting where junk mail
handling is *not* enabled.

Will it turn off junk mail handling for people that have it turned on?
Meaning stuff that is somehow ending up in their junk mail folder will
then start ending up in their inbox? Will any email in their junk mail
folder disappear, or will it stay in that folder or end up moved to
their inbox?