Few remarks to answer that:
you can not flash different GB sizes, for obvious reasons
it seems you cannot downgrade
downgrade is not only going from PR1.1 to PR1.0 but also supposedly from a higher to a lower number in the firmware code (..._legacy_480_ARM_... to _legacy_232_ARM_... won't work, the other way around may...)
from one colour to another works fine (done that)
from one region to another also works (seen plenty of reports on that - mind you, some regions have different language support, supposedly different preinstalled software, etc)
don't know if you can install oem versions, I wouldn't since I don't want to end up with a telecom-branded phone

I don't know if it's wise to disable OTA updates, but I recommend to use Navifirm to get the latest if not offered via OTA and if the update will get you improvements you want/need. For me it was Swype, which wasn't there in PR1.0.

I will wait for OTA for PR1.2, since I haven't seen anything in PR1.2 that I can't wait for, and with navifirm updates you have to reinstall all apps... which shouldn't be necessary with OTA updates.