Oh, this is fun.

It is a practice for some FF’ers to pay the fare difference to full first/ business/ economy class when things are looking difficult (storms, major snafus, whatever) to ensure that when reaccomodation starts, they are higher up the pecking order, and that their ticket is as flexible as possible

Note that in Europe, flying Business class is often the same seat as economy (or used to be) The differential is all in the flexibility of the ticket, including being looked after better when things go pear shaped.