Your attempt is close, but there’s a small adjustment needed to properly set the text to uppercase. You need to use the .eval() method to get the current value of the parameter, convert it to uppercase, and then assign it back to the parameter.
Here is the corrected version:
op('combined_text').par.text = op('combined_text').par.text.eval().upper()
This script retrieves the current text from the combined_text operator, converts it to uppercase, and then sets the parameter back to the modified text.