What happens if DUC expires?

Does anyone know what will happen if a DUC expires before it has been paid?

Normally the DUC is active (can be paid) after 48 hours and then has a grace period before expiry. There is an alert that states (for automatic renewals) that the renewal request has to be made again if the payment was not made within the 8 day required timeline:

If it expires, will the opportunity to renew be available again or will I have missed my chance?

Need to understand the background to your question better.
Have you already generated the DUC (ie gone online and generated the DUC according to your Card Number), but cannot pay just now (ie perhaps it needs an account in Portugal or some other practical constraint)? If this is the problem - ie a practical ā€œhow best to payā€, my understanding is that anybody in Portugal (including your lawyer) can do this for you within the 8 days, and be reimbursed if you cannot do the payment directly.
What I am uncertain about is whether - if you let the pedido (request for renewal) lapse too long - whether the opportunity will still be there in a month or more - if you are NOT one of those whose card will expire within Q1 of this year! I do not think anybody will know the answer to this question.
So it might be a good idea to try to find a way to pay as early as possible, if you can.