VS Code: How to convert snippet placeholder to uppercase or lowercase?

Try this:

"test": {
    "prefix": "test",
    // "body": "${1} -> ${1/(.*)/${1:/upcase}/} > ${1/(.*)/${1:/downcase}/}"
    // simpler version below works too
    "body": "${1} -> ${1/(.*)/${1:/upcase} ${1:/downcase}/}"
}

You need to hit Tab to apply the transformation.