This only works for strings that have a single comma in it. And it does not know whether the replacing is sensible or not.
E.g. "Peter, Paul and Mary" would become "Paul and Mary Peter" (also refer to Emerson, Lake and Palmer, Earth, Wind and Fire ... I don't want to think about "Dave Dee, Dozy, Bicky, Mick and Titch")
Anyway:
Create an action of the type replace with regular expression for the field ???sortorder (should be artistsortorder or bandsortorder):
Search for:
(.*), (.*)
Enter as replace string:
$2 $1