Originally Posted by
ElPippo
What about upgrading with cash? Does the same rule still apply?
No, the same rules do not apply for cash. In short, you can use cash to buy-up from any fare class except for E, basic economy, (which cannot be changed).
Using cash to "upgrade" is not really an upgrade, rather, it is buying up to a new class of service, and they will issue a new ticket and cancel your old one. However, unlike when you change your ticket to a different flight, they will waive the change fee because you are buying up to a higher class of service on the same flight. They will apply the value of your V ticket to the PS ticket and you will pay the difference.
One other option: If you prefer to use miles, you can ask about buying up to an eligible fare and then use miles (but it may be a better value to just use cash).