Can you change recovery mode from simple to full with users working on the server?
You can - as long as there's no running transaction on that database, or you run the alter statement using WITH ROLLBACK IMMEDIATE. Be careful with this clause as it will rollback nonqualified transactions - see here for a human-readable explanation.
Also, remember to take a full backup after you change the recovery model from simple to full, or the change won't really take place (info here)!