i'd just hijaack ubbthreads.php, do a query on that field after your get the $user object
if NULL then slap $ubb='editbasic' and fall thru
that would force them to answer yes/no before _ever_ going on
now, if you really just want that _one_ field edited, _and_ you want to kinda give then a nice reason for doing so, then you could create 2 files ( assuming english only board)
scripts/onetime.inc.php
templates/default/onetime.tpl
where the tpl kinda explains why they landed there in nice / friendly terms
the forum has one field ( radio yes/no ) and a submit.. it posts back to itself, updates and then sets $redirect to forums display and done
