This is something that was written to assist in changing text files in a scripting process. Simply type TEXTCHG /? at the DOS prompt for a list of options.
The enclosed utility called BINCHNG was written about the same time to allow a user to alter a binary file with the same byte count but with different data.
An example of this using the TEXTCHG.EXE file, if you type "BINCHNG TEXTCHG PROGRAM" at a DOS prompt, it would change all occurrences within the TEXTCHG.EXE to PROGRAM. So when you then typed TEXTCHG /?,it would show PROGRAM on the help text that appears, instead of the TEXTCHG that normally appeared.