MONTEREY IS RUNNING. (It may be "up and" as well)
I swapped my 1 TByte SSD for an "original" Apple 128GByte SSD and updated the computer to Monterey. Swapped the drives again and then restarted the computer but had a flashing folder symbol. Nearly made the mistake of re-uploading Big Sur but stopped the process when I saw elsewhere that this wiped the data too. (The Apple site didn't mention this but a Youtube video did).
Fixed the issue by running repair disc then pointed "startup" at the correct partition. (I think it was confused as my drive is partitioned whereas Apple drive was not)
Updated to Monterey.
Drew McFarlane 