First I'll say that there is nothing wrong with the first generation P99 pistols. They are as accurate, reliable, and durable as the later generations. The proprietary rail didn't bother me, and if it did, I'd buy a rail adapter from a member on the Walther forum that sells them to 1st generation P99 owners who want to mount lights and lasers on their pistols. The "ski-hump" in the trigger guard (which was put there to stop gloves from going under the trigger) never bothered me either. The improvements made in the later generations were when Walther went with the S&W rail, and started to polish the feedramp and areas that effected the smoothness of the trigger pull. I have over 7k rounds through my 1st generation P99, with only one malfunction due to a 147gr round of MagTech that had no gunpowder in it.
Second, the P99c wasn't introduced until 2004, when the second generation was introduced. There are no 1st generation P99c pistols.
As to the OP, I would make the trade, even though I much prefer the AS trigger to the QA trigger. Walther's polymer pistols are some of the more overlooked and underrated pistols out there. Lots of people didn't really like the QA trigger though, so make sure you do.