Unfortunately it is not possible to change a username on the account. The reason? Changing the username would make it impossible for you to access or update any of your cards.
This is because all transactions are cryptographically signed, encrypted and stored on a blockchain for security and audit. Changing the username would unfortunately change the cryptographic keys and this would orphan your current customers.
We do appreciate the inconvenience a fixed username may cause, but in the future, blockchain functionality will open up opportunities for you to integrate with other loyalty programs (E.g. exchange your full cards for airmiles, or use credit card points as currency).