I had a trip last month that returned to MEM the first day of a 5 day trip. Concerned I might be eligible for SUB, I reviewed my contract on the flight back to MEM.

The very first sentence of the SUB section says, "Applicability
Substitution provisions apply only prior to block out on a trip. Substitution shall not occur due to Company actions made for operational reasons (e.g., trip revision, high minimums pull) after block out."

Does that answer your question?