If you qualify for the credit for part of the year then you are entitled to it for the whole year.
I would advise making sure that it is taken off your credits for next year before the new certs issue however, otherwise you'll get it for the first few weeks and then it will be withdrawn and clawed back.
If you are really willing to wait until January to move in then I reckon you could probably qualify next year too by waiting until a bit into January.
Not sure if Revenue can do anything about it, pretty cynical in my view, but people used plan around the tax year when to get married back when it mattered, so.....
Sybil