ah I see - There's no history of production when capturing a city. When you take a city, you review the production, but it is reset to nothing and you make adjustments from there. There's no bonus for producing what was produced before. So you don't get to piggy back on production time already spent. But if a city is marked as a specialty for a certain unit type, that specialty is not lost on capture. So you would still get the percentage advantage/disadvantage depending on what you decide to produce there.