So we're getting the Special Ash Hat Pikachus again for Sw/Sh.
So quick question! how about the ones we already have from Us/Um? Can we transfer them over?
Yes, they can be transferred.
Source: https://www.thegamer.com/pokemon-sword-shield-how-to-get-all-forms-pikachu-raichu/#:~:text=There are several unique forms of Pikachu wearing,iterations can be transferred to Sword & Shield.