I had asked Mindpearl to check again, and she sent me this:
Here is the answer from our Headquaters in the USA, regarding changing the first coupon on your ticket.
"For a change before departure after ticket issuance there is no charge for reissue but all other rules apply including the general rules.
As with any other type of fare, when the originating flight coupon is changed, the ticket is to be exchanged for a new ticket at current
rates and following all current rules.
So, even if changing from a LONE4 to another LONE4 (without penalty), all current fare levels and rules of the LONE4 must be followed.
Anytime you change the outbound flight, the original ticketed contract is invalidated; in this case, a reduction of maximum segments from 20 to 16."
So it seems that our "Headquaters in the USA" concurs with the rating desk in Dublin.
It's not that you can't change the first flight; you can, and they accept that there will be no change fee, but it will be reissued under the new rules (which means only 16 segments) and, it will be at "current fare levels".
Which could be much worse than a change fee, given the way rates are likely to go!
So one way or another, I'd better get myself to JNB for Jan 28th!
BTW I have been very impressed by [employee name removed] at Mindpearl; very responsive to emails, promptly issued tickets, and dispatched them by UPS when she said she would.
All at a time when she must have been unusually busy.
Happy Travels.