Well, not exactly.
You can do all the modification you wish to a percussion revolver (keeping in mind automatic weapons restrictions), permanent or otherwise, and still sell it with no penalty as long as it does not use modern cartridge ammunition. You can shorten or lengthen the barrel, reshape the grip frame, change the hammer, install a plunger in place of the hand and many other modifications including modification of the frame for a loading gate.
If, however, you modify the gun to shoot modern cartridge ammunition (eg, installing a cartridge conversion cylinder), then you cannot sell the gun with that modification installed without a federal excise tax license. You may, however, remove the modification, sell the gun as a percussion revolver and then sell the modification separately without the tax license.