I think that if you do the upgrade at a later date, it's possible that there may be some extra taxes to pay. Typically, this is because the sterling equivalent of an overseas tax has changed because of exchange rate movements. But these changes (and any additional collection) are usually very small.

Is is possible that what you read was a reference to this? If so, I wouldn't worry about it - and sometimes these changes actually result in a small refund to you.