If the KN2600 do format the card properly, and if you can save and read any files at all using the keyboard, it should "see" the update file as well.
But, SD card size is pointed out in the procedure, so it may as well be a point to follow.

I do remember there was some dicussion about the the limit / max storage capasity of the SD cards usable at Technics.
As years goes by, there has been a huge step forward within digital storage as well as "bits and bytes". ( 8, 16, 32, 64, 128 bit and so on)