I'm throwing out my theory now: Yui had a goddess to begin with, but it switched to Keima when they switched. It's a decent explanation for why she remembers the feelings but not the events, and his face when he realizes he had a goddess all along will be epic.
Especially if they have to try and strengthen that goddess with love. Wonder who would do the job best? If Elsee is still filling in for Kanon at that point, it would most likely have to be Tenri or Haqua. Personally, I can't decide which would be funnier: the tsundere trying to romance the Conquering God, or Diana's interference.