@Mark1976 If you don’t want to lose any days you have on the current subscription, please contact support and have them add the days to the new subscription, otherwise you will lose those days.
In an older there here was my suggestion to another user with a similar issue:
In the event you must do each device separately the screenshot below helps with doing that for you.
First click in the area where the arrow is at the top of the photo, the information you need will appear below it. Hover your mouse over the words Serial Number and two dialogs will appear just to the right. You can enter the new product key there manually if needed.
SA