Rough process & time converting Residential to Office.

You will need to speak to an Architect that works in your area; you may fall into “Loft Law” issues with your residential tenants, you may not be able to remove AIR space.
But with all the additional office space being developed in new purpose built buildings and the lack of need or want of older office building space in Manhattan…. why are you thinking of converting to office space? The costs of doing so as well as the Real Estate assessment changes etc may make it not the best use.