Can I Upgrade my CPU in my Lenovo Thinkpad T400?

I currently have an Intel Core 2 Duo P8400 and I want to upgrade to an Intel Core 2 Duo E8435 or an Intel Core 2 Duo T9900. Is this possible, or will I just ruin something? I have read that someone was able to change the stock CPU to one from a mac. If neither of these work, what should I get to upgrade my speed to at least 2.8 GHz?

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