Downgrading 1.1.2 and 1.1.3 question

I was wondering the following:
I can't understand why when somebody accidentally upgraded from 1.1.1 to 1.1.2, they could downgrade to 1.1.1, virginize the baseband and the iPhone could be unlocked and activated with no problems. This thing about not being able to downgrade the baseband was there since 1.1.2 right? Since iPhones that came with 1.1.2 preloaded can't be activated. I thought it was the bootloader that came with new phones that did not allow a baseband downgrade. So, why is everybody here saying that if you upgrade to 1.1.3 you won't be able to downgrade and unlock and activate your iphone even if you have the bootloader that came with 1.1.1 or earlier?

for the same reason the 1.1.2 couldn't be unlocked

You an only upgrade or install the same version of baseband on 4.6BL

Unless you have apple's secret key of course, then you probably could downgrade...