I don't think that changing to a different fare class is part of the same day confirmed option. Ticket changes can be done anytime prior to departure. And main cabin (classic) tickets don't have change fees. One wouldn't be expected to purchase a full Y fare on the the desired flight(s) if that's all that is available during "same day confirmed " window.

Same Day Confirmed is $75 flat fee and only available if the same fare you purchased is available on the new flight. If the same fare is available, you are technically not eligible for Same Day Standby and must confirm the earlier flight for $75 or take your original scheduled flight.

So if the fare class that was originally purchased is NOT available on the new or desired flight(s), then one should be eligible to be put on the stand by list and stand by for the new flight(s).

Once someone is within the "same day confirmed" window a particular "fare class" may or may not be available on different or new flights (ie V, Q, K etc).

With my experience with Delta phone agents several times, I've been offered to be put on the stand by list for the new or desired flight(s) if same day confirmed option is not "totally free." The $75 fee is waived for me being a platinum medallion.