Thing is, 3:1 is appropriate in far more tunes than 2:1, so it ought to be the default, if there is any default at all.

The issue isn't with "replace" vs "update" but with *what* was going to be replaced or updated - I assumed it was a file on the site, not the contents of a box on the screen. You need to be a bit more explicit about what your service is actually doing. (I'd assumed it retained every tune ever submitted to it somewhere, though I hadn't gone to look).