Most .455 conversions seen are to .45 Colt. And there are several different methods to do that. Conversion to .45 ACP would somehow involve increasing the space between the rear face of the cylinder and the frame by shortening the cylinder. Or possibly replacing the .455 cylinder with one made for .45 ACP. Not sure if that replacement would work or not. The increased headspace would make the use of .455 cartridges somewhat iffy because the .455 cartridge has a very thin rim. It might or might not work, I am not sure. For use with the .45 Auto Rim or .45 ACP without moon clips, cylinder modification would be simpler and it could still use .455 cartridges. From your description, I suspect the rear cylinder face has been machined shorter to accommodate .45 ACP in clips and .45 AR, and may not work well with .455 cartridges. No matter which, the effect on value would be negative. It would be just another shooter grade revolver.