I have two comfort bikes that I use for general commuting.
#1 Diamondback Wildwood Citi
#2 GT Timerberline
Both habe pros/cons, etc. I was curious as to the ability of changing the semi-flat bars to drop-down bars. Obviously this would require a different shifting/breaking setup. Currently, it is the twist shift style. Both bikes have the 7 speed shifters, the has the Shimano Revo SL-41 twist shifters, whereas the Diamondback has The SRAM and is waaaay out of tune.
If you don't have to have drops, I would just put a set of trekking bars on the bike. That will give you a lot more hand positions than a standard flat bar. Nashbar makes a good one, I think it is on sale right now for $14. Check the coupon forum for additional savings.
The first pic is the Nashbar product, the second is the Dirt Research trekking bar I bought off of ebay.