If you can’t change your router’s SSID and passcode to match what it used to be (maybe it is a network someone else controls instead of you, or whatever else), then you don’t have to “Delete” the devices to add them to the new network, but you do have to go through the setup process all over again.
As long as you don’t delete the devices first, it should maintain most of the rest of the settings, including any rules/routines, etc that were associated with the device, so you don’t have to redo everything, you just have to go through the setup process to add the new network and name the device again (you can rename it the same name, but you don’t have to).
Just going through Setup again (without deleting the devices and all their other settings first) is the second-best way to do it with the least amount of work, but if you can do what @Sam_Bam suggested, what he suggests is the easiest way without having to go through set up on all your devices again.