How can I downgrade my Nook NSTG from 1.2.1 to an earlier version? topic

Dear Community,
I have a NSTG with US 1..21 Version installed.(no root). I want to install the UK Version 1.2.1 because it has other languages aswell. In order to do that I have to install an earlier version than 1.2.1 on the nook so that it will detect the 1.2.1 UK version as the correct one.
Has anyone an Idea how I can downgrade my Nook via SD card or ADB?
Thx.
FSG