David:

Technically, you are absolutely right, but for practical purposes, it really doesn't matter that much. DL doesn't inform a purchaser that this is really a coach ticket with a confirmed upgrade unless one bothers to read the fine print of the fare basis, which most people never would do. The way they present it on delta.com, a reasonable person would think they are buying a FC ticket. You can't use fine print that most people will never read to counteract a false impression that you have provided in the main presentation.

Therefore, if you are on a P ticket and get downgraded (which can certainly happen if there is an equipment change), you will either get rebooked on an comparable alternative itinerary in First or else get compensation (if they don't offer you compensation, you should write and demand it, as you would legally be entitled to it).

This helps to explain why DL always downgrades FC rewards tickets (purchased FC ones, not upgraded) first, regardless of the status of the passenger. That way, the only compensation they need to give is in Skypesos, which they don't really care about, not in cash.