Short answer: no. Changing the macro (the #define) will not change the underlying code. Changing the number will.
Long answer: Most of the .h-files and some of the .c files contain a lot of lines starting with #define. These are called preprocessor macros, and are primarily a reading/coding aid.
When you compile your code, it is actually run through a series of steps. At first, your code is passed through the preprocessor, then the code is compiled, and lastly it is linked. During the preprocessor step, #define's are replaced, literally, in the codebase. So, all commands are transformed from "ACMD(do_stuff)" to the #define'd replacement, "void do_stuff(struct char_data *ch, char *argument, int cmd, int subcmd)". This substitution is literal. So, your CLASS_MAGIC_USER will be translated to the number it is defined to, before it is ever compiled.
I am curious, though - you realise that changing the MAGIC_USER macro doesn't alter what the players see? To alter that, look in constants.c.
