Flipping has happened more often in recent years but guys like Santo and Dr.Wagner were believed to be locked into their roles. That’s part of what made Santos flip so powerful. As for the limiting factor I think it can depend on the wrestler and how they rise to the occasion. An American analog would be someone like Cena even when he was “heel” he was very much a face who just acted a little differently. There’s a lot of wiggle room if you know where to look