Switching the inner chainring on a triple crankset from the factory 30 tooth to a much smaller 24 tooth can be done with few or no changes to the bike. And no to minimal changes in shifting. Assuming its a "road" triple crankset, it will use the 74mm bolt circle diameter for the inner chainring. You can buy this size inner chainring for $5-$10 mail order. No reason to pay for a name brand. There are never, ever any shifting ramps on inner chainrings because the chain just falls onto the top of this inner chainring or gets lifted off the top to go to the middle chainring. The inside side of the middle and outer chainring may have shifting ramps to improve shifting.
The only changes you may need is the rear derailleur. The "mountain" bike rear derailleur will have a longer cage so it can wrap more chain and keep it taut in more gears. If you use the inner chainring, 24 tooth, with the smaller cogs in back, the chain will hang loose. The longer cage "mountain" bike rear derailleur will keep the chain taut in more of the smaller cogs. Whether the chain hangs loose is not really a big concern, but its possible a loose hanging chain could get wrapped around something or jump off the pulleys. In reality you most likely will only use the inner chainring with the bigger cogs on the cassette when climbing steep grades so the hanging chain will never really materialize. But something to keep in mind.
STI shifters slam the chain from the middle to inner chainring in one rapid movement. No finesse to them. Bar end shifters and Ergo shifters have several ratchets that allow you to slowly move the chain down to the inner chainring. With STI's rapid forceful shifting style, it is likely you will drop the chain onto the bottom bracket shell more often. Particularly with a smaller inner ring because the chain has more free air to fall in before it hits the top of the inner ring.
To prevent dropping the chain, get a Third Eye chain watcher. Or the N Gear Jump Stop chain watcher. I use them on all of my bikes, except the fixed gear of course. Cheap insurance to protect the paint from being chewed up by the chain if you drop the chain. Below are some links to the devices.
http://www.bikepro.com/products/chains/chainretainer.htmlhttp://www.peterwhitecycles.com/accessories.asphttp://www.cambriabike.com/ShopDisplayProducts.asp?ID=20&cat=Chain+Guides/Transit/Protctrshttp://sheldonbrown.com/harris/chains.html#deflectors