Getting a new iPhone? You can transfer your Signal account including message history. Set up your new iPhone with Apple's Quick Start but be sure to *skip* wiping your old phone until *after* you've transferred your Signal account from within the Signal app.

https://support.signal.org/hc/en-us/articles/360007059752-Backup-and-Restore-Messages

Backup and Restore Messages

Signal messages, pictures, files, and other contents are stored locally on your device. If you have your old device, select the platform to transfer messages: Android iOS Desktop Message restora...

Signal Support
@signalapp Switching between android and iOS is still not possible :(
@cent @signalapp
Not beeing able to migrate Signal is the only thing that impeds my complete switch from iOS to GrapheneOS. I want to leave the Apple cosmos, but I am locked in (and can't even blame Apple on this) ! :-(