I have justoved to a plus account. That happens to my original Monzo card and card number. Say I have my card details with a few merchants, will I need to change the card details for all to the new plus card number ??
Or would the best result be to have a virtual card for the like of fast food takeaway and other merchants.
You’ll have to update the details to either you new card, or a virtual card.
If you update to the virtual card now, you won’t need to wait until the other card arrives
Although there is another thread about auto card detail updates 
It’s not mandatory to get a new card. I upgraded to Plus and still retain my Hot Coral card.