Best thing to do is contact Now TV's help chat, if you explain your problem to them they are likely to extend the expiry date on your 6 month pass.
However, as I am a customer like yourself I have no direct knowledge of what they will actually do. But I have, in the past, bought 2 x 3 month or 3 x 2 month Entertainment passes and had them consolidated into 1 x 6 month pass with a new future expiry date.
The point I am making is that Now TV's help chat has the ability to amend pass dates in order to help their customers.
Unfortunately I have now cancelled my now tv broadband. The chat I went with and completely wasted my time. The person actually asked me what they should do. They then suggested I either loose my paid 3 month pass or my 6 month pass. I suggested an extension of my current pass or a code instead of the activation email. Very annoyed and very disappointed with the customer support. Even when I went to cancel all I got was an ok that’s done for you.