Cap and Ball to Cartridge?

Yes. This was done in a couple of ways, and the question of cartridges had to do with Remington's patent on them expiring in the late 1860s. Early cartridges were rimfire, with centerfire coming along later.

The first practical conversion for the Colt was the Theur Conversion. It was slow to reload, though faster than cap and ball, and made in small numbers. Next on the list was the Richards-Mason conversion, which added the ejector slide on the side of the barrel that later became one of the visual trademarks of the Peacemaker/Frontier family. The Richards-Mason was done in .44 for the 1860 Army and .38 for the Navy Colts (both the 1851 and 1861).

Smith & Wesson, if memory serves, was the first company to come out with purpose-built cartridge pistols. They used a break-open hinge to speed reloading. I'm away from the books or I'd give you precise dates and model names/numbers for all weapons. PM me if you decide you need them.

And as a side note, all of these weapons used black powder. Smokeless didn't become common until after 1890 or so.

If I remember right, the Thuer system used an odd system of loading and extraction. It didn't have side lever at all, but retained the loading lever that was original to the Colt Army. It loaded from the front of the cylinder using rimless cartridges. You then had to rotate a special ring and drop the hammer to eject the cartridges out the front of the cylinder again.

The Richards-Mason was much closer to what we consider the "standard" Colt.

With the S&W, I think time and availability was a bigger issue than cost. The Thuer came out first.

Conversion to black powder shells was common, what you can't do is convert to modern smokeless powder. The guns weren't designed to take the increase in power.

Actually, while S&W was the first *practical* cartridge handgun, with the 1858 Model 1, a seven-shot pocket pistol in .22 rimfire short. (the gun I'm holding in my profile pic is a 1861 Mod. 1, 2nd issue); the Europeans had developed the pin-fire cartridge a few years before. The S&W gun was based upon the 1855 Rollin White patent for a bored-through cylinder, which they combined with the metallic cartridge in a gun with a "tip up barrel" such that the cylinder could be removed entirely for loading. They later produced the Model 2 "Old Army", a scaled-up long barreled gun in .32 long rimfire (which never quite caught on with the military being considered underpowered). While several other manufacturers tried to copy the design during the civil war, S&W sued them all into oblivion... so most major gun manufacturers simply waited for the patent to run out in 1871. SEE: http://www.armchairgunshow.com/SWTU-info.html

Most handgun manufacturers spent 1869-71 working on cartridge firearms to get into the market... Colt and Remington, having massive inventories of black powder revolvers and parts produced for the civil war quickly produced "conversions" that replaced the black powder cylinder with a bored-through one, and some form of loading notch or gate. Later models conversions included adding an extractor to deal with sticking shell casings. However, such a conversion did not require factory tooling...any moderately competent gunsmith with a well-equipped shop could convert a black powder revolver into a cartridge weapon. And such conversions were vastly cheaper than a wholly new firearm, often costing 1/2 the price of a gun entirely designed for cartridge use. I've seen a wide variety of such conversions from the 1870s and 80s, ranging from the ever popular Colt & Remington Army and Navy revolvers in .44 & .36 cal... To such pecularities as Colt Pattersons, pocket pistols, Starr Double Actions, and even a LeMatt. Some indistiguishable from factory in the quality of workmanship...to others crudely modified with little more than a changed cylinder and a milled groove through the rear sheild & grip to allow loading.

Incidentally, modern conversion kits for reproduction black powder shooters have made a comeback among cowboy shooting hobbyists.
See:
http://www.armsportllc.com/colt.htm
http://www.armsportllc.com/remington.htm
http://www.armsportllc.com/conversions.htm