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Great great question. From my experience you'll be looking at a roughly 2 to 8 week hit in rankings.  I know that's a wide margin but it depends on more than just the single redirection of your homepage on the city domains to the city page on the global domain but also includes the content and of course the internal links. You'll want to insure that you move any content that's attracting traffic, social signals or links and also redirect the pages with links or traffic to the appropriate page on the new domain. Past that it's really just a matter of waiting and trying not to chew your nails off. I wish you luck, that's a hard call to make but I think you're making the right one. Regards. Dave