The change in commanders in Afghanistan is probably very good. It should sharply increase our chances of succeeding in what America wants to have happen there. Success is possible, but it may not be easily recognized as "winning."
As one who well remembers the Vietnam war, having been commissioned into the Army then and having studied why we got into it in the first place, the real problem that led to the build-up to half a million troops was Gen Westmoreland. He was a conventional war General fighting an unconventional war, and his solution was - more troops - more troops. The book and movie "We were soldiers..." and the battle it depicted showed the genesis of his obsession. America won that one battle of attrition. But the strategy it led to caused us to lose the war.
Westie mousetrapped LBJ by demanding a massive increase in troop strength that allowed him to fight a war of attrition against the Victor Charlie. Westie's timing was perfect. It was when LBJ wanted to pass Medicare and the Civil Rights Bill. Had he not given Westie his troops, LBJ knew that the conservatives would have shut down the Congress and LBJ would have passed zilch. So Lyndon gave Westie his troops to pass his bills. Lyndon knew we couldn't win in Vietnam when he did it.
Shift forward to the Afghanistan conflict. Again, a conventional general fighting an asymmetric war, and his solution has been what? More troops. Conventional war generals don't like the strange apparent lack of discipline of Special Forces. They never have, as the Rangers will tell us from WW II.
Bringing in the Special Forces Lt. Gen. Stanley McChrystal is critical to winning in Afghanistan. There is no alternative to winning there, as failure in Afghanistan puts the Pakistani nukes at risk. Leaving a conventional forces general in command is too great a risk to accept