1. Have the original creature have an interaction that it uses on its opponent. This interaction grants the opponent the ability to resurrect/animate your original creature and apply an immediate body transformation syndrome into said smoke creature.
You could also maybe give the creature a secretion that acts like evil biome fogs for resurrecting and transforming very specifically that creature's own corpse. So when you cut them down, or whatever, their secretion will infect the corpse, resurrect it and transform it. You could hide it in a secretion that they would already reasonably have, like sweat or blood or tears or something, or you could tie it in to the smoke monster theme by making them just breathe smoke and stuff.
That way, if it works, adventurers (and I assume you're making this for adventure mode because it's Bogeymen) won't be able to cheese it accidentally or intentionally by not using the interaction.
Though do be aware that bogeymen with the associated bogeymen token will not leave a corpse, so if you're using that token then neither of these solutions can work as there won't be anything to resurrect.
You could alternatively not give the commander NOFEAR, and make it so they are likely to flee at some point. Then give them an interaction they only use while fleeing that turns them into a fearless smoke monster. They'd transform before dying, though, and there'd be no guarantee that they'd transform at all.