In addition to modifying the character map, you also need to change the name of the font. Two fonts can't have the same name. So the modified version needs its own name. The program produces the name automatically by adding a suffix to the family name. I recommend using OSF for OSF fonts, SC for SC fonts, and EXP for expert fonts.
Fonts have several different names.
The program attempts to handle them all consistently. If the names aren't consistent, the program should produce an error message. In such a case the program will have to be fixed to handle the pattern of names present in your font. The last two arguments are the input and output file names. The input should normally be a font file directly from Adobe.