You don’t need to upgrade to PRO version. The download link of the free version and the PRO version are exactly same. You can install POS on two computers or more with full features even if you download from free version link. You will get the first monthly bill about one month later if you have more than one user or exceed the limitation of the free version. If you don’t get the bill, just enjoy the free version. You will have 15 days to pay after you get the bill by email.