You can boot from NAND, mount the SD card, and make the change required.
Check the environment variable settings that you need:
This thread shows the environment variables that we use for SD booting (not certain if they are the same settings you are using), but also highlights the differences between the settings required for SD booting and the original settings.
So change back to the original settings, boot from NAND, mount the SD card, make the change you require, power down, and revert the environment variables.
You should be able to configure for multi-booting (from SD card if it is present, else from NAND) using techniques mentioned in
this thread.