It doesn't matter what the UK CAA says or doesn't say, it no longer has control over the law. Article 4 of EU Regulation 1178/2011 states that JAA licences are to be deemed EASA licences and all are to be replaced by 8 April 2017 - 5 years after the law came into effect. Therefore, a JAA licence issued prior to 8 April is replaced at the end of its 5 year life. So far as those licences issued during the transition period are concerned, all must be replaced by April 2017 and so a JAA licence issued today will have a life of a little over 4 years and 7 months. The maximum period by which a JAA licence's life could be shortened would be 5 months (assuming that it were issued on 7 April 2012), which is hardly significant in the great scheme of things.
There is nothing in law that requires JAA licences to be replaced by Sep 2014 (unless they were issued before Sep 2009). In fact, there is nothing at all significant about Sep 2014. The UK's decision to delay implementation of Part-FCL until Sep 2012 did nothing more than shorten the transition period, which still ends on 8 April 2014 and by which time all non-JAA compliant licences must have been converted.