No charges for non-maintenance of AMB in Signature Banking. However, downgrade is built-into such programs in case of continued non-maintenance of sufficient balance.
Can the downgrade happen?: Yes, that's how it works.
Will the downgrade happen?: Not necessary.
In any case, the downgrade will never be a surprise. You will be notified well in advance so that you may replenish the balance.
Hope this answers! Cheers!