The circuit has not changed, just the PC boards.
The SEX Kit C4S active load upgrade should work just fine. The one currently available is for older version of the stereo SEX amp, not the most recent 2.1 version. This is embarrassing, but I don't actually have the manual for this and I don't remember whether we increased the plate current or not. You may or may not need to get a pair of 237 ohm resistors for R1.
But I would wait (if your amps are working OK right now) until we release a version of this for the SEX 2.1. This version will use the latest green board, version 4.4. I think (hope?) this version finally does everything we need and we'll be gradually transitioning all C4S applications to this board. One of its advantages is that it can use either of the two power transistors we like, and there is room for a clip-on heat sink on that transistor. Less heat = more longevity.