iMessage - change the sender number in existing conversations
I have 2 numbers, and I have set them up on 2 different phones, but I only want to bring one with me, so I set them up so that I transfer all calls from one to the one that I carry, and to receive messages on both devices, but only alert on the one I carry.
Now I'm facing an issue where I want to make sure that messages that I send out, are only sent from one of them, as that's paid for, so I want to prevent sending messages from the one that I do not carry with me.
The problem is with ongoing / already existing conversations: I found no way of changing the sender on those, and I have tested this. There is only a setting called "start new conversation from..." but that doesn't cover existing ones.
How can I go about changing the sender number in existing conversations without deleting those conversations before?
Thanks.
iPhone 6, iOS 8.4