Bart,
regarding timing I'll let Don have the final say. However, I talked to Don about exactly this issue at the social, and it seems to me the best time would be as soon as possible *after* we've done the 4.6 merge on head. That merge should happen within a week.
The reason for making the change soon is that head is supposed to grow increasingly stable as we approach summer. On the other hand since we have so many major new features that we want to go in we will accept the risk of destabilizing head initially.
I want to point out here (as Don has maybe done already somewhere) that 4.6.3 is supposed to be purely a bug fix release (no new features).
