An additional domain is any domain whose price is not included in the package price. The package gaining the domain as well as the package losing the domain must both have the same Customer ID. If the packages are under different accounts, you have to transfer a domain from one account to another instead.
Transferring an additional domain from one package to another is free. The next bill for your domain(s) will occur at the time of the domain renewal or expiration date which will remain unaltered by the transfer.