Originally Posted by GagaPilot

I believe this specific clause comes into play if you were to be involuntarily downgraded by DL. If you were forced to fly Main Cabin, DL would refund the fare difference based on date of purchase (original ticket date).

For a voluntary change, the best you can do is to cancel and take a credit, and rebook at current rates. In this case it’s too close in to get a Z fare.

Basically just because a flight has Z inventory, doesn’t necessarily mean you can switch into
it without honoring the fare rules, in this case the advance purchase requirement.