There is no way to convert. You have to go back onto web site and register as commercial and pay the $5.00 fee. It is screwed up, you can't even change your email address without re- registering and paying the registration fee again.
More government inefficiency!!!!
Remember, you never actually registered your bird as a hobby flier. That registration was for you and the number could be used on as many aircraft as you wanted. Under Part 107, you are registering a specific aircraft and each one gets a unique number (I currently have 2 registered and getting ready to submit the paperwork for the third). BTW, it takes a little longer, but I went the paper route to get "N" numbers for my bird just like those that manned aircraft have.
There's some information here if that helps. Don't pay anyone to do it for you. It's quick and easy but there are places out there charging to do it and making it sound like they are doing more than they are.