We're rolling out Windows 7 by attrition. As old machines reach end of life, we replace with Windows 7 machines. We're no longer downgrading to XP. That is how things like this go. Eventually we'll be replacing the Windows 7 machines with Windows 8 (or whatever it eventually becomes).

Unless at some point a corporate initiative comes down where someone higher up says, "we're going to have hard lifecycle periods with complete system refreshes at that time, corporate wide". Some companies go all in, some do it a step at a time.