Hello,
I made the same kind of mistake (i'm using SxOS 3.0.5)
I was on CFW 9.1 (exFAT) and I've upgraded the firmware on 10.2 (FAT) using ChoidujourNX.
==> result : no longer able to boot on cfw from ExFAT SD Card.
I get a new SD card on FAT and i've copied on it last boot.dat from (SxOS 3.0.5), SxOS Directory , licence & 10.2 firmware.
==> with this 2nd SD Card from ChoidujourNX i've upgraded the firmaware from 10.2 (FAT) to 10.2 (exFAT).
But now when i'm putting back 1st SD card (exFAT), from SxOS menu when i choose to boot on Emuland : I observe Nintendo Logo then black screen.
Is someone can help me ?