You'd need a fresh install, since the hardware would be very different. I wouldn't recommend even trying, as I've seen folks do similar things with in theory identical hardware and the drive wouldn't boot on the new, and not on the old when moved back.
Is there a way to replace the windows installation on the SSD with a fresh one without erasing any of the data on it? Or would I have to move the data to something else
Not short of backing up the data on another drive. Unless you want to futz with the most core basic drivers that windows will require in order to fully boot. If you don't know how/want to push drivers via CLI, throwing your porn onto an external and then installing windows on the hardware itself works a jiffy.
If you can find a way to connect an SSD to the deck, I’m not sure if it’s SATA or whatever, if it’s connected and recognized, I don’t see a reason why the deck couldn’t boot from it. Not speaking from personal experience, all hypothetical, I could be wrong. Worth a shot if you understandably don’t want to do a fresh windows install
Hold the volume down button while booting to get to the boot menu. If you get a bootable drive connected, it should show up there. You could connect your windows ssd and it should work fine.
However, the steam deck needs its drivers for the GPU, Bluetooth, and similar. If you plan on swapping back and forth, that may not play nice. Installing portable windows to an SD card really ended up not being that hard and I'd be happy to link you to a guide if you needed one.