What if someone buys the wrong membership subscription?

Because MMM does not currently allow members to either cancel or upgrade their current membership, this has to be rectified by the membership secretary.

This can be achieved in a few steps:

1. Go to the members details in Manage members and remove their membership product
2. Credit the members account by the amount they paid for it (you can see this by looking at their transaction history). This should be done as a refund against the membership product (not a "credit by other means")
3. Ask the member to purchase the correct membership, whilst reminding them that they have credit on their account (from step 2) that could fully, or least partially, fund the purchase (they can choose to use their club credit to purchase a membership)

Important note: Once a membership has been in place for more than a day, the amount that should be refunded is discretionary. Thus MMM makes no attempt to automatically refund the purchase which is why step 2 has to be performed. But the only exception to this is if the removal is done on the same day as the purchase. Then, because the membership has only just been bought, MMM can safely assume that the full amount is refundable and thus it does automatically issue the refund. In this case only, step 2 is not necessary as the system has done it for you.

Alternatively...

You may want to replace the membership without bothering the member to go buy the correct one. In which case perform steps 1 & 2, but instead of asking the member to buy the proper membership you can apply it yourself to their account.