I read somewhere that you won't get dinged for the Costco/AMEX card going away because it's the company canceling, not the user. If Citi buys the data and issues cards based on it, you'll have the same credit limit and age on the card, so again no change to your FICO score.
If they don't buy the data, I'm guessing you will get a ding. I had a card closed for non-use and the drop in credit limit did drop my score.
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I thought one of the factors in the FICO score was length of time with individual credit cards... so if you've had a costco amex for a long time - it could ding you... even more if it's your oldest credit card.
I don't think it will have a huge effect - but it will have SOME effect.