Also, under "Additional Rules" in the same link, note "Medallion Complimentary Upgrades do not transfer to your rebooked ticket"
So you have to put it all together for them:
- A confirmed RUC is a "premium cabin fare".
- By contrast, a Medallion Complimentary Upgrade remains booked in the original (non premium cabin) fare, which is why they "do not transfer to your rebooked ticket."
I asked this question years ago (and have an email from DL confirming it). In that email, I was told that the relevant information is "also available in 'KM', the online information system we use. The specialist would go to Same Day Travel Overview, then "Newly Desired Flight Requirements" and it's under the 3rd bullet point."
Don't know if the references to KM have changed.