Vanilla PR1.3 can be flashed on top of a branded PR1.3. That's been proven over and over again. For instance my N9 from Dubai was flashed with Northern Europe PR1.3 (considered vanilla) from PR1.2.1, then reflashed with PR1.3 from Dubai when it finally came out. Not a problem.

You have a point, downgrading isn't possible it seems - I may be wrong. But if you're talking about going from branded (like Telcel) to vanilla, that's very doable as long as they're either an upgrade or sidegrade (same to same).

But if you're talking about a branded PR1.3 to PR1.2, not possible from what little I know/have seen.

But why would you want what you're asking? "Just because" or "I can do it on the Lumia 800" isn't really a good answer. Both are minor players as compared to other mainstream phones and the ability to do such a minor thing - convenient or not - is really like splitting hairs. Nice if you can do it, but can your Lumia 800 be upgraded to WP8? Can it be unbranded, unlocked and then have USB OTG or mass storage enabled?

In the end, your point doesn't matter. Both phones/platforms will invariably be forgotten to the average customer by January 2013.