Three scenarios.
1) Moto releases ICS. Most likely build 247. From there, we upgrade to JB. The most likely scenario.
2) Moto ditches 11 builds of ICS and goes to JB directly on top of 905. There is almost a 0 chance of this happening.
3) Moto pushes a patch to 905 that will allow JB to be installed on top of GB. This is the one you should be worried about.
Scenario 1...no problem. We're all covered. Scenario 2....not as big of an issue even with new security. Higher security means I can push it with the HoB so not a problem. Scenario 3 is the one that'd screw us. Unless they pushed higher security with, let's say 906. If it's the same security as 905 and they pushed another GB build, then those on 235+ are screwed for quite some time. Not a likely scenario, but, if they want to abandon ICS for JB that's the likely way it'd go down. And it'd be ugly if it did.