Assuming that A..Z and a..z are contiguous in the character set (not guaranteed, but for our purposes we can consider it to be so for all practical platforms), 'a' - 'A' will give you a transformation necessary to convert to lower case.
For example, if 'a' is 98, and 'A' is 65, then adding (98-65 = 33) to a character will make it lower case.
You're better off using the standard function tolower; it will get the nasty details right in case the characters aren't contiguous etc. as Kaz pointed out, although as he said for all practical intents and purposes they will be.
Which muds did you play in the 90s, kaz? There was a kaz that fits your personality on a small one I frequented.
Probably not me. I played with my full nick back then, Kastagaar. I was mainly on Land of Legends (not the one whose implementor is on these forums, the old one that closed), and occasionally A Moment in Tyme. I also implemented for Continents Mud (which, I'm happy to say, is still around. If anyone plays it, say hello to the crew from me!)