Perhaps I'm not smart enough for this. After decades of designing online user interfaces and several hours perusing the PP website, I have still not identified a single link or menu item labeled "UPGRADE ACCOUNT" (or anything remotely similar). So, if we already have a personal account, how do we initiate the process of upgrading it to a business account? Is there a list anywhere on the PP site that spells out exactly what information is required to upgrade an existing, new and unused personal account to a business account? For example, does it require anything more than a DBA document that has been filed with one's state government?
As you have not added your country to your profile (so we can see when we hover over your username) or said in your post which country you are in then sadly can't advise. Paypal varies a great deal country to country so you need to tell us where you are on this worldwide community forum.
Advice is voluntary. Kudos / Solution appreciated.