iMessage: send from new number in ongoing conversations In iMessage, I'm receiving at 2 numbers (and a couple of emails), but I would like to ensure that all messages sent out are only sent from one of them.

As in its name, the setting "Start new conversation from..." will only work with new conversations, not with existing ones.

How would you do this without deleting the conversation?

EDIT: Another important question: how can I find out what number I'm sending out from on an existing conversation?
Sadly, you can't change the conversation that the number is coming from without deleting the conversations.

Also, the only way to tell what number or email a conversation is coming from is to ask your friend what number it is coming from. The way they can tell what number it's coming from is by either looking at the control bar at the top of the screen or, if they have that number in their Contacts, clicking details, clicking the "i" next to your contact then seeing what number of yours (or email) is in blue text instead on black.

I know this is complicated and I wish there was a better answer to this question, but sadly Apple needs to do more work on the Messages app.