The operating system is installed on the SSD. If you swap it, there is no OS that the Steamdeck can boot from and you get an error message to that effect. So you have to reinstall the OS even if you did not use it before.

I would also recommend to use the Steamdeck as it is for a few days, before upgrading the SSD. So you can make sure that everything works and you do not have to RMA it (I imagine a RMA is more complicated/impossible if you have swapped the SSD).