I think it depends on your carrier if there's certain network features requiring a new SIM (AT&T required a new one for VoLTE during the iPhone 6 era, but an older SIM would still work while generating reminder texts). If nothing is changing, there's no reason moving your existing one over shouldn't work.
These phones will still have a nano SIM slot and presumably work like prior iPhones if your carrier doesn't support the internal eSIM (see the recent cellular iPads). If you order through a carrier, it may come with one preinstalled and "staged" to replace your existing one.