When you’re within 5 feet of an ally on your turn, you can expend one superiority die and switch places with that ally, provided you spend at least 5 feet of movement. This movement doesn’t provoke opportunity attacks [...]
The grappled condition states:
The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.
Assume a monster with a 5-foot reach is grappling an ally; the Fighter is adjacent to the grappled ally, but 10 feet away from the monster. For a picture format, F is the fighter, A the ally, and M the monster:
What happens if the Fighter tries to use Bait and Switch on the grappled ally?