Priority was given to private pilot licenses (PPL) and instrument ratings (IR) with the expectation that the annex would eventually include commercial/corporate pilot licenses (CPL), and airline transport pilot licenses (ATP).
Recognition of an ICAO IR by EASA/JAA/National NAA's and the process to obtain a Part-FCL PPL has been in place for many years in a number of EU states and you will note that in order to 'convert' an FAA Instrument to a Part-FCL IR(A)
With regard to the demonstration of theoretical knowledge and as detailed in the IPL, if the applicant has a minimum experience of at least 50 hours of flight time under Instrument Flight Rules (IFR) as pilot in command on aeroplanes, he/she shall demonstrate to the examiner before the assessment of instrument flying skills that he/she has acquired an adequate level of the required theoretical knowledge, in other cases, the applicant shall pass a written examination within the 24-calendar-month period preceding the month of application.
Understanding what is currently required for those Part-FCL IR(A) holders who seek to revalidate an expired rating and comparing that with the theoretical knowledge level needed to pass the FAA Instrument written, I would be interested to learn how this might be achieved.