A Mollie account is tied to a specific company, with a specific Chamber of Commerce registration number. When the legal form of the company changes, you are also issued a new Chamber of Commerce registration number.
If any transactions were already received into your account, it is not possible to just simply change the legal form. In this case, you need to create a new account with us.
Go for more information about creating a new account to: How do I sign up for Mollie?