Changing text to uppercase is a specific thing that would have to be built-in to a specific piece of software.

If that isn't a feature built into Quickbooks already, there's nothing in Windows that will automatically do it for you, secret-keyboard-shortcut or not.

Your best option is to figure out a way to export the data into an editable file format, use a third party application to perform the transformation for you, and then re-import the data.

Second option, which is only a slight step above retyping everything manualy, is to copy (ctrl-C) each field out of quickbooks, paste (ctrl-V) into a third party application with the switch-to-caps feature, then Ctrl-C and Ctrl-V it back into quickbooks. Depending on the nature of the text, it may not be that much keystroke savings over just retyping.