According to 66.B.100(b), the competent authority shall verify that all required modules of Appendix I to Part-66 related to that new (sub)category sought are met. Credit can be granted as regards to the basic knowledge of the (sub)category for which the licence has been already issued.
This means that a comparison of the basic knowledge (gap analysis) will have to be done between the different (sub)categories. Such a comparison has not been yet done once and for all in Commission Regulation (EU) No 1321/2014 due to the different potential cases (wide diversity of (sub)categories).
Therefore such a comparison should be done by the Member State that has already issued the Part-66 licence, before the applicant is enrolled in such a “gap” basic knowledge course.
Please contact your competent authority, which may have already performed this comparison. Some competent authorities may have already posted such comparison(s) on their websites; however be sure that the comparison relevant to your case is acceptable to the competent authority who issued your licence.
In addition the applicant will have to comply with the additional experience requirements for the new (sub)category sought, as detailed in Appendix IV to Part-66.