Lanefan
Victoria Rules
Ah - I was assuming the same genre, just different worlds or planes. For example, jumping from 5e Forgotten Realms to 1e Mystara not just in setting but in rules at the same time.Only if, as you say next, that the referee runs all the mechanics. Otherwise the players are likely to revolt. Still, suddenly taking the perceived security of the rules away from the players could still lead to a revolt of sorts.
That's describing black box play. It's one common mode in FKR gaming, which is my preferred style. But selling that to the players is a big ask. Most don't want to deal with it. They get a sense of security from knowing the rules. As if the rules will somehow protect them from a capricious referee. They can't, of course. But...players.
If the goal is to gain a sense of genre-mastery, then sure. You'd want to settle in so the players could get a sense of the genre's "rules" in game...as if the players themselves wouldn't already have a sense of the genres involved. Unless the genres involved are a surprise. But that would only last 1-2 sessions at most. Seeing a superhero flying through the sky would give it away, as would a zombie apocalypse, or the sudden appearance of Cthulhu or dozens of bodies and their murders to solve. If the goal is the fun of genre-hopping, it would be fine.
I was also assuming this wouldn't be a forever change, just a side trek in a longer ongoing campaign. So, for example, you'd start out in 5e FR for a year or two, then due to in-game reasons jump to Mystara, adventure there for six months using 1e rules, then jump back to FR (5e rules) and carry on from there.