I am still kind of lost on what the post was trying to address

You have a business class ticket for a flight; you wish to change to another flight where business class is not available and so your main option is to pay for 1st class which has a fare difference of $2300

If upgrading a fare , you pay for the higher fare basis and book onto the flight; there is no need for there to be business class availability when paying for a 1st class ticket

Upgrading a fare is quite different to using FF miles to upgrade a booking