I obviously only have checked this theoretically, because I don't have serial access to my device but:
* 5.6.x will most likely prevent downgrading.
* Hell, it might even break Active Content on the PW2. (Or it might *fix* Active Content on the KT2 & KV, who knows).
* It probably breaks some stuff (you'll need a snapshot version of the SS hack, for instance).
* It currently prevents any custom update files from running, no matter the state of your JB.
So, yep, not touching that one with a ten-foot pole until someone with serial access checks it out in practice

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