Can I combine message conversations from one contact's different phone numbers? My friend recently got a new phone number assigned to the same iPhone she's had for a while, and sent me an iMessage from it to let me know. Since I didn't have the phone number already, it of course appeared separately in my iPhone's "conversation" list from the messages we had sent back and forth using her old number.

I've added the new phone number to her contact info now, and it would be nice if I could keep the two conversations together -- just continuing with the new number from where the old number left off.

I've found a program called Merge available on Cydia, which looks like it would do exactly what I want. I am not particularly interested in jailbreaking, however. Is there any way to accomplish the same thing on stock iOS 6?
On stock iOS, different contact numbers or email addresses are given their own "conversation". This answer still applies, even on iOS 6.