If you EF someone who already has the HoT running, it just overwrites.
I tried switching out SS for EF, and I'm quite happy with it. Although in some cases I really miss the massive heals of a regular WoG, but then it's usually because somebody else screwed up badly, and I'll just have to use other panic heals to save their bacon.
I've tried using EF with Divine Purpose, but the procs seem a bit too unreliable for my taste so I'm considering switching to either Sanctified Wrath or Holy Avenger. I'm mostly leaning towards Sanctified Wrath though. Downside is that it's a 3 min cd, but 50% reduced cd on HS, combined with EF, sounds nice. I love Holy Avenger as Retri, but as Holy I just have trouble generating HP fast enough as I only get that from HS and HR, and I'm having severe mana issues so my use of HR is quite sparse. :/