Once you have purchased the SIM there are no further charges due so there is no need to update the payment method on your Mobal MyAccount.
If you do update the billing information this will not be a problem, but note that our system will automatically perform security checks on the information provided. This will include an authorization against your credit card for a small amount (usually ¥100). Please rest assured that this is only an authorization (this will disappear from your statement within a few days) and we will not actually take the money from your account.
Please note if you have other Mobal products (that are post-paid) then it is your responsibility to ensure that the billing information we hold on file for you remains up to date for that product. Please see the support pages for the relevant product for more information.