This is an entirely new idea for me and one that I believe should be approached with an abundance of caution. The idea of doing this just because 'it's that time of year' or something akin to that, is very scary. I'm all in favor of the concept of allowing people to participate in deciding their assignments, as long as it is in the interest of the employee, the team, and the company, but I think doing this 'just because we can' could be quite disruptive.
Timing could be very critical, and I would definitely do this is stages as much as possible, even to the point of making the transitions as gradual as possible to maintain some overlap. It could be much easier to 'undo' a single transition that to fix several at the same time.
Maybe it's just my lack of experiencing this.
One of the best days of my IT career was they day I told my boss if the problem was so simple he should go fix it himself.