No, it is replacing the stock with custom characters. It works the same for C64 and Atari:
1. Put your custom charset bytes (8 bytes per char) somewhere in RAM. The traditional slow way is a bunch of data statements and a loop to POKE to some location.
2. Tell computer the new charset location
For step 1 here, I save space and time by positioning the actual string at a spot in memory that I tell the computer the charset is located. I fill the string with special chars to represent each of the 8 bytes per custom char. Therefore no DATA, looping, POKEing required.
This works in Atari since a string can be thousands of bytes long, unlike C64 which must be 255 or less. I create a dummy string that takes up useless ram but positions the very next string right where I need it. This was a typical Atari BASIC trick from back in the day.