Its from the BMAA Microlight Instructor and Examiner Guide and it refers to differences training between either microlight control type, or between SEP/SSEA and microlight. Thus, strictly speaking, it doesn't necessarily apply to the situation proposed by the OP of going for the NPPL M first and then "upgrading" his licence to be able to fly heavier stuff later. I take it to mean that differences training to fly a microlight (of whatever control type) must be to a standard commensurate with a GST pass on a microlight (which, as GtE pointed out, is not the same as e.g. for a LAPL).
Apologies if I've further muddied what are already muddy waters
