I would have thought that the obvious first questions are for pilot.90:
Have you a preference where you want to do your training?
Have you had a Class 1 medical certificate issued yet (by an EASA member state)? - This may limit where you do your training as not all EU member states have embraced EASA Part-FCL yet and may still restrict training to the State of Licence issue.
Is your TC licence a CPL with MEP IR or SEP IR?
Knowing this info would allow anyone to give better advice as to the requirements for conversion.
In any case, if you want to fly EASA aircraft commercially in EU airspace you WILL need to do all 14 theoretical knowledge exams, regardless of experience or ICAO Annex 1 licence held. There are validation routes available for persons holding non-EASA ATPL to fly EASA aircraft commercially for limited periods but a non-EASA ATPL can only be validated for one year (and thereafter extended for a further year - MAXIMUM - but can only be extended if the applicant has commenced a course of study for the theory exams). A validation will only be issued once.
