It would be easier to do it before splitting them but if you were to do it from the split data, the easiest way to do it would be to
Open the editor and select Promotions
Delete Smackdown Live
Rename RAW to WWE
Start a new game
Hire all the Smackdown Live Roster.
Yes. That is the easiest way as that requires a lot less clicking. On the plus side, you could weed out the workers you don't want anyway.
Not 100 on this but I don;t think you can arsenic contracts of workers who aren't in your roster already, though there are other ways of doing it simply enough.
__________________________________________________________________
Ended up not having time to do this over the weekend so I might wait until the February update drops and use that as a base. I will do my regular split data with ALL companies still involved. Not 100% sure if I'll go on to do a new WWE Universe split as that involves deleting every worker that does not work for WWE (which is as exciting as it sounds). While doing this, hopefully I'll remember to put aside a split game where RAW/SDL are still WWE and have NXT/UK/205/PC set as separate companies for @Oakesy