[shilala]

Since this is all hypothetical, and I don't think Mo would go anywhere, nor would the Yankees remotely entertain losing the value of the media circus and merchandising surrounding his retirement, I'll just play along for fun...

He's retiring, so it'd be a good deal for the Yankees.
I don't think it'd harm his legacy or his Yankees legacy at all.
I don't think a sixth World Series ring this year would even make me feel that Mo "went out on top." He IS the top. Another would just be another ring. Nothing special (you know what I mean), and not really a cherry on top of his career.
Going to another team wouldn't make me think any less of Mo. Yankees fans would likely b1tch, but who wouldn't?

All in all, looking from every direction I can think of, it'd be a bad move for the Yankees and Mo, both. That's from a fan's perspective.
From a business perspective, if I were the Yanks and the numbers crunched, I'd trade him if he allowed it. I just can't imagine a trade would be more lucrative than a massive retirement celebration.
Mo has become a media darling this year and opened up dramatically. He'd be an absolute star come post-season.