I know, but I think people need to be warned that despite what they might assume, "flex this fare" doesn't necessarily book into Y with the expected upgrade priority. Nor does it always give the cheapest flexible fare. In fact, it would be interesting to do some experiments on markets with refundable K fares.

Also, I've noticed that delta.dumb sometimes gives a lower fare for exactly the same flights on the same dates when you do advanced search from the start with the refundable/flexible box checked compared to picking flights and then checking the "flex this fare" box.

If you want full Y, it's better to search from the beginning by fare for Y or higher with the
refundable/flexible box checked on advanced search.

The bottom line is that you should never purchase the "flex this fare" option without investigating whether you can find a better ticket on delta.dumb.