Does the butterfly keepsake's buff persist after you swap it out?
No, you lose the benefit from the Butterfly as well as the Lambent Plume if you swap them out for another keepsake. Although the Lambent Plume does have a side benefit of working as a pre-requisite for Hermes' Legendary Boons, so in some cases it can still be worth taking for only one biome, but there's no such thing for the Butterfly.