You won't need to go through UAC and you don't high grades to get from com/law to commerce. You're simply dropping the law component of your degree, so you should be able to freely drop back down to commerce. See the Student Information Office and they'll advise you on the process.
When I changed from Science / Law to Science, there was no paperwork involved, and the Faculty of Science organised the whole thing - I'd go talk to the Commerce Faculty Office and see what they say.