This whole subject can be quite confusing so here goes.
Since September 2003 when EASA came in to power all CofA’s have technically been EASA CofA’s, whether they where still the old CAA type or the new version referring to EASA, but the UK system of a three year CofA renewal stayed in place.
From September 2008 all aircraft have to have an EASA CofA and an Airworthiness Review Certificate, the ARC. This is the transfer point to the new EASA system in total.
In order to implement this the CAA starting reducing the three year CofA to Two years in 2006 and One year in 2007 so that all CofA’s would become due for renewal between September 2007 and September 2008. At this renewal you would get the EASA CofA and an ARC and be ready for the EASA starting point.
From September 2008 to September 2009 all renewals would follow the EASA process, and a full ARC renewal will be required (i.e. a Star under the old system) because the EASA approved Management Company (the CAMO) can only renew ARC’s that it has issued and the first ARC will have been issued by the CAA. From then on the ARC renewal will be every year or three years dependant on how you have your aircraft managed/maintained.
Thus, in the worst case scenario, those who had a CofA renewal between Sept 2006 and Sept 2007 will also require one in 2008 and 2009.
I hope that this makes it a bit clearer.
ACX