I want my old iMessage to my new iPhone

If you have a backup from a previous device that contains the messages you want, you can restore that to the new phone. Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore. You can't access a back and only restore certain files.

Backup - Restore your device from an iCloud or iTunes backup.