What's the goal? Is it important to drop the column, or are you only looking to change the PRIMARY KEY constraint? If the latter, you don't have to drop the column, you could simply delete the PRIMARY KEY constraint and rebuild it. If you don't force a change to the clustered index, it shouldn't be slow at all.....
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?