Change the letter case of the cells you selected — UPPERCASE, lowercase, Title Case, Sentence case — free, and fast enough for thousands of cells at once. Pro adds editorial Title Case, camelCase, snake_case and more.
One permission, nothing else. It only ever touches the cells you selected in the spreadsheet you have open — never numbers, dates or formulas — runs entirely inside Google Apps Script, and sends nothing anywhere — no server of ours, no database, no analytics.