I cannot access my old iMessages

Hello x5115,

Thank you for using Apple Support Communities. Congratulations on your new iPhone 15 Pro! If you're not seeing your messages, we might be able to help out. Follow the steps below:

1.) If you were syncing your messages with iCloud, you're going to want to make sure that you have this feature enabled and turned in order to sync your messages: Set up iCloud for Messages on all your devices - Apple Support

2.) If you were not syncing your message with iCloud, they will be included in a backup that you created. If you used a computer or iCloud, you can restore from that backup to recover missing data: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

We hope this information is useful. Thank you for using Apple Support Communities. Take care.