Me thinks best to create two partitions on the new disk, one for (a fresh install of) windows, and one for BoX and DCS.
That's what I'd do... And probably, you will need MS's help regarding the licensing...
There is a trick you could try - sign in with your or a Microsoft account to the current Pc, to 'register' the current W10 license with that account, then swap motherboard and stuff - boot from the current disk/OS, login with your Microsoft account again.
Next, put in the SSD and disconnect the 500GB disk, reinstall W10 from scratch, login with your Ms account. Later, hook the 500GB back up. Now I am far from sure whether this would work, but I just needed to share my creative thought process here