I was checking the list of supported operators and noticed there was a toLowerCase() but no toUpperCase()?

I was starting to consider an approach to create one by looping each character when it hit me….

  "elmType": "span",
    "txtContent": "@currentField",
    "style": {
        "text-transform": "uppercase"

It’s easy to forget that there might be really simple workarounds instead of making things complicated!