Need help switching one conversation from SMS back to iMessage!

I accidentally switched one of my conversations to SMS yesterday when the phone "suggested" switching (because a single message was having trouble sending). So far, I've tried deleting and re-adding the contact into my messages (losing our conversation history), restoring my phone from a back-up, and switching iMessages on and off in settings but nothing has worked. All of my other iMessage conversations are still the same, and that's the case on my friend's end as well. It is only our conversation that won't go back from SMS to iMessage. Help! These green bubbles are killing me.