Forget about that lat xfer/redesignation board stuff sovbob posted. Doesn't apply to his situation, seeing as how both SWOs and SWO(N)s have the same designators in the fleet.
If your husband is really deadset on SWO(N), he most definitely needs to do two things: speak with the SWO(N) folks at Navy Personnel Command, and speak to someone at OCS about his situation as well.
All I can tell you is that there isn't exactly a shortage of SWO(N)s right now, which could make convincing the Navy to grant him an interview a little tough... hope this helps.