When my wife and I were first dating she had a business trip to San Francisco. I confirmed her upgrade on United even though she hadn’t given me the flight number or record locator.
I knew the route she’d be flying, and when she got back, so it was easy to pull up her flight details and get her reservation changed. I used miles — United only charged 10,000 miles back then and no cash co-pay — rather than a confirmed upgrade certificate because those certificates were paper back then and I wanted to surprise her (not hand her a certificate and explain what I was doing).
She told me later that she had to decide whether she thought it was sweet and awesome — or creepy — that I’d altered her reservation without telling her.