I've heard many different explanations for why Michael replaced Tom with Vito for consigliere.

The first one I've heard was Vito was dying and getting old and Michael wanted to use his wisdom as much as possible.
The second one I've heard was Tom wasn't a wartime consigliere, but his face of disappointment when that was announced almost convinced me he had the ability. Or did that mean he was vulnerable to getting attacked?
But the best one I've heard was how he was testing Toms loyalty, especially after the betrayals in the family.

What do you guys think? Tom is one of my favorite characters.

There is a little truth in all the theories you espoused. There is an ongoing sibling rivalry between Michael and Tom that went back at lease as far as December 7 1941 when Pearl Harbor was attacked and Mike signed up for the Marines. At that time he was livid at Tom for discussing his future with Vito. There is also a foreshadowing of Michael's concerns about Tom as consigliere during the wedding scene when he tells Kay that Tom is in line to be consigliere. He makes it a point to say he is a good lawyer, but not a Sicilian. In fact
Tom was not a wartime consigliere. He was always trying to settle things and compromise. Santino complained to him about this and so did Michael. In the end, I think Michael trusted him but he was always testing him and using his position to belittle him. Still he trusted tom with the fact that he was going after the heads of the five families, how to deal with Geary and how to settle things with Frank Pentangeli.

Basically, I agree with DT. However, Michael also blamed Tom for not protecting the family: Sonny's murder and Vito's near murder. That's why Tom was not a wartime Consigliere.

As DT points out, Michael tells Kay at the wedding, Tom will become Consigliere although he's not Sicilian (he also unnecessarily tells Kay Tom's last name although that may be a way for Michael to inform the audience about how Tom became a family member). In II Michael tells Ola that Tom only handles certain parts of the family's business. Later in II Michael reveals Tom's having a mistress. Of course, the flashback scene says it all: "You talked to my father about my future ... my future."

Tom and Oli, very true, I can see it now. I always found michaels and toms relationship to be blurry. Sonny's and toms was my favorite one though. Or the dons love to his son Michael.

I agree that the best answer is probably a bit of all three explanations - although strictly speaking, Michael could have kept Tom as consigliere, at least officially, and continued to receive all the counsel from Vito that he needed, and thereby have the best of both worlds.

Fundamentally, however, I think it's because Tom in fact was not a wartime consigliere. Tom seems to have been a superb analyst and strategist. But he was basically a gentleman by temperament - notwithstanding the dirty deeds he'd done. He simply didn't possess the killer instinct of the Corleones.

Yes to all of the above, plus one more:

Michael was planning for the family to move to Nevada. He needed Tom out of harm's way--and in no way associated with the violence to come--so he could function as the family's legitimate front in setting things up in Nevada.

Also, don't forget that the peace Vito made was about to be violated and Tom had been part of the peace making. That's why Vito told him "there are reasons why you must have no part in what is going to happen."

While I agree that all the theories above have merit, I think a major factor not yet mentioned was Michael's personality.

It was not in Michael's controlling nature to work in consultation with someone else.

He doesn't actually replace Tom with Vito as consigliere, he basically eliminated the position.

His words are telling: "If I ever need help, who's a getter consigliere than my father?"

Yes W, that statement from Michael was telling. As II & III illustrate, Michael apparently did eliminate the consigliere position.

At least Tom was a "peace time consigliere." What the hell was George Hamilton?

He was hired to consult Michael on a new venture: opening a chain of tanning salons.

Michael wanted to have his own appointment in that position, and felt the need to distance Tom from the illegal aspects of the organization and instead focus on getting them deeper into legitimate businesses. He did not want Tom as a lawyer representing their legal businesses to be tied to any crimes.

In the book it explains why he did what he did to Tom but also in the book he put him back in the position after everything was settled. Two was strictly movie and so was 3. I'm going to look through the book again brush up on what happened. Been a while.

Tom was taken out form being Consigliere for a few reasons.

1. Things were going to get rough, don't want him being hurt.
2. He isn't a war time Consigliere, sure he can fight a war but if you want to use the skills of tom Hagen efficiently send him to do the legit family business.
3. When your planning something like they were the less people who know the better. Even someone they trust like tom. Although he figured it all out anyway because although he wasn't a Sicilian Vito had made him one.
4. I always thought a part of it was, tom was with the don when he made the peace so in a way tom was a party to the peace.

Michael did have complete trust and confidence in tom Hagen, you can see when the don has died and they are about to go to war he makes tom hang around. Pretty much reinstalling as Consigliere.