I have two Apple IDs, because one of them is in the US App Store and the other is in the hungarian Store. I use almost every service of iCloud on the US account except for the iMessage. I still have to log in to iMessage with my older, hungarian account, because all my messages since the service was introduced belong to that. I want to simplify my life and use only the US account (except for purchases in the Store). Is there any way to transfer my old iMessages from my old account to the new one?
Short Answer: No
Longer Answer: Apple does not support and even don't know how to merge 2 Apple IDs/Account. But on your local iPhone you can hold all the Data if you only switch the iMessage ID afaik. iMessages are only hold for 14 Days in the Apple Account and are not/nerver longer stored at Apple (claims Apple)
So moving iMessages to another account without Dataloss - No Keep old Messages on the same Phone (or Backup) - Yes