GUID format change when converted from a string

The Guid itself is actually a 128-bit integer value and the string is a hexadecimal representation of the same value and hexadecimal is not case sensitive.

Guid.Parse("429637FA-C124-4a37-8DBD-09F103419313") and Guid.Parse("429637fa-c124-4a37-8dbd-09f103419313") will be equal:

      Guid a = Guid.Parse("429637fa-c124-4a37-8dbd-09f103419313");
      Guid b = Guid.Parse("429637FA-C124-4a37-8DBD-09F103419313");
      bool equal = a.Equals(b); //true

If you want to compare the string representation of a GUID with a System.String, you can use the ToLower() or ToUpper() method:

if(guid.ToString().ToLower().Equals(s.ToLower()))
...