This can be done by using a customer import.
Customers-> export customers. If you do not want to have all the customers exported then after pressing export customers there are some filtering options. Downloaded file should be modified leaving only these customers that need to be changed to a company. It is important that customer ID is left to the import file.
Then open customers-> choose import customers and select modified excel or csv file. Continue mapping and choose right headings -> customer name should be mapped as company name.
