>>58530639>regenerator user needs to spend at least one full turn in battle for regenerator to be able to activateIt's literally that simple. Alternatively, you -could- make it so that it heals 25% rather than 33%, but both changes at once would be overkill.
You know how speed boost doesn't just trigger because you switched blaziken in? It will only gain the boost at the end of the next turn. Regenerator should be the same. If you want 33% HP on switchout, you can't just brainlessly go back and forth between toxapex and amoonguss to get them from the brink of death back to full health with no downside. They will need to spend at least 1 full turn doing whatever, and then and ONLY then will a switchout heal them. This implies flip turn alomomola can't just flip turn immediately, it NEEDS to waste a turn wishing or protecting (even when it doesn't want to) if it wants the flip turn recovery. Same for teleport/chilly reception slowbro or king, or any of the u-turners. In turn, your opponent can work around your new need to stay in when they predict you'll want the recovery.
It's disruptive enough to momentum to reduce regenerator's usefulness just the right amount without nerfing it to the ground.