You can create an action of the type "Replace" for ARTIST
(see FAQs /t/967/1 on that)
Enter as search string: ;
(which is actually semicolon-blank)
Enter as Replace string: /
(which is actually blank-slash-blank)
Adding slashes to fields may play tricks: the slash is not legal for filenames. SO if you want to use the Tag-Filename converter with %artist% as part of the mask then the slash will be removed.
Also: the Windows Mediaplayer sometimes interprets the / as field separator, so that you get a group call ac and one called dc instead of AC/DC.
So: be careful with that - the semicolon was not so bad after all.