It took me 2 phone calls and 2 chats this time around to get a rep willing to help me.
I haven't previously seen the credit option Amazon offered me today, so I would like to share it here.
Below is the rep's side of the chat:
We do not have option to extend your promotional credits .
However, we can make an exception and remove those credits and add $42 under your Gift card and it will never expire.
I am sorry but we are not getting option to add Gift card.
Shall I issue refund of $42 from which you got charge for your Prime membership?
I have issued refund as promised and the refunded amount will be credited to your payment method for your Prime membership within next 3-5 business days.
Bottom line, you have to get a rep willing to help because they have to manually remove every credit and add it manually elsewhere.