When you log in to your new store account, you may find that previous licenses you have purchased in the past have been grouped together, giving you a single upgrade price for two or three licenses. If you only want to upgrade one license, you need to ungroup them to give you a single license purchase price.

This is a 5 seat license, and it is giving me the option to purchase a 5 seat upgrade with associated pricing. However, I only want to upgrade one seat. To do this...

At the top of your My Products tab, there will be an information banner telling you that you have multiple licenses. Click on the button "Group Identical Licenses" and select "Do not group

Now when you scroll down, you will find your licenses have been separated, and you can upgrade for a single user price, by clicking on the "update addon" buttons.
Note that when you next visit your account, any remaining licenses that are identical will have been regrouped, but if you upgraded one license, it will now remain on its own as it is no longer identical to the others.