Well it's clearly a legal issue, since to do the M->SSEA upgrade his needs a current licence, so he needs to revalidate (presumably by test) on 3-axis microlights before he can do his SSEA GST. Daft I know, but it's the rule and other people have been caught out on it.
This is no longer the case. After consensus agreement within industry had been achieved, I changed it earlier this year. Please see:
http://www.nationalprivatepilotslice...20REV%2009.pdf
This means that, in the case of Microlight to SSEA, the applicant must have had a valid Microlight Class Rating in the 12 months before the date of application for the SSEA Class Rating. The exam must have been passed in the 24 months before the date of application for the SSEA Class Rating and the SSEA GST not more than 6 months before the date of application.
Incidentally the exam policy was agreed because practical 'differences' would be covered by practical training; however, AC Gen & PoF is a good check of essential knowledge, so that was the only one we deemed essential.