The rule is once you have played a match in RGL for a regiment then essentially you are committed to that regiment. But I will add we decide these on a case by case basis (which is also in the rules). So if someone joined 55th, played 1 round at the end of their last match then they disbanded, its not really their fault and its a shitty situation for them so we would be willing to make an exception.
Obviously, as long as the other reg leaders are okay with it.
Imo some of the rules are a little contradictory and aren't really that useful but at the end of the day I have come in to host RGL, not EIC etc. so will follow on whats been working.