Unfortunately, InfoPath doesn't have the maturity of something like Visual Studio and C# when it comes to text handling. It would have been great if we could have had access to a .ToUpper() or .ToLower() method when dealing with text, but alas, we do not.
To that end, we have to use a workaround leveraging the translate() method. In order to generate the equivalent of .ToUpper() which converts all text to uppercase, you'll need to use this:
translate(., "abcdefghijklmnopqrstuvwxyz", "ABCDEFGHIJKLMNOPQRSTUVWXYZ")
Vice versa, to get a .ToLower() equivalent which converts all the text to lowercase, you'll need:
translate(., "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz")