I wonder how they control that? Usually it will be a free schedule change (no rerouting, no charge) but other times changing a transit to a stop will violate a rule, and the airport or res can't be expected to know which.

I've (theoretically - haven't seen the tix yet) planned a transit that cannot be turned into a stopover because I will already have used the two I'm allowed. Will they have printed "no stopover permitted" on the coupon or something?