If it's important to him I imagine it's something that can be put in writing. He probably has more control over playing 2B with the Mets than the Mariners.The choice is his as to whether or not he wants to go, I can't see him moving to third and being happy about it.
And while not finalized, the pieces going to Seattle have been public for a while now.
I know the players involved have been reported but not all of them are going to be included in the trade, from what I read. Time will tell but as with all things Mets, I begin hopeful with an expectation of getting screwed.