A couple months back, I gave up on my iPhone 6 (too unwieldy) and began using my old iPhone 5s. Regrettably, I didn’t restore the 5s from a backup, so its SMS history began as a clean slate. Two months later, in preparation for the timely iPhone SE release, I find myself wanting to merge the two SMS histories. Unfortunately, no such way existed, officially or otherwise... until now.
With some Ruby/SQLite hackery, I managed to create ios-sms-merge - you’ll need to download the kludgy iBackupBot to import/export from your iOS backups, but with persistence and this script, you can now combine your SMS histories.