iMessage: Contact has new number, keep messages within same thread.

Hi all,

My partner has gotten a new number yesterday. I assumed I could just add her new number to her contact in my phone, remove the old and keep our text thread going with all the photos, links etc with the new number now.

At first all was fine, old thread disappeared and all was within one message thread. When ever I close the iMessage app and reopen it, boom old thread appears.

If I add the new number to contact but keep the old number (with the new number having the star beside it on the contacts info page) it creates two threads.

Any advice would appreciated. Just frustrating having to go back out and go to another thread to find an old link or photo.

Cheers.