If you are talking about a period conversion (C&B) to cartridge you are looking at considerable work. The back end of cylinder the has to come off and several significant parts installed. You can go to www.uberti.com
and look at their 1860, 1851, 1871-1872 conversions and get an idea of what is involved.
Midway is selling an R&D gated conversion that has the cylinder, and loading gate for $224.99. You will also need the installation hardware - another $39.99, and the ejector assembly (price ???).
If that is what you are wanting, you can probably buy the finished product and have a warranty cheaper than you could have it done.
If you are talking about a 1st Generation, 2nd Generation or Signature Series Colt BP, you could easily destroy a nice collectible.
Final Caution: It was a practice during the C&B era to carry extra loaded cylinders. This has even been shown in a few movies over recent years. Carrying loaded cylinders outside of a gun is extremely dangerous. They are easily dropped and unlike a cartridge that will burst if detonated outside the cylinder, the cylinder has the same potential as the gun to launch a projectile
Hope this helps.