This happened this weekend, and we were unsure how to resolve it.
Normally, I could not target the Progenitus because it has Protection from Everything, but since it lost all abilities and became a frog, it was a valid target (We thought so anyway). It was at the end of this turn that we were unsure. Polymorphist's Jest effect ended, but the Darksteel Mutation was still enchanting the creature. This is what we thought:
- Progentius has Protection from Everything and the Enchantment "falls off" as it is no longer a valid target.
- The Insect has no abilities apart from Indestructible, and the Enchantment stays.
- Something Else???
I'm really hoping this is #2 since this combo would allow for targeting of things with shroud and hexproof, etc. on the turn I cast Polymorphist's Jest.