I booked a United itinerary (a revenue ticket) using Chase's travel portal. LAX to ATH, then TLV to LAX, two pax, all in P.

Due to some changes in covid policies in the countries we were visiting, I am trying to just change this to a round trip LAX to ATH ticket. I found an itinerary on Chase, which is also a United itinerary, the only difference being the ZRH to ATH leg is in Y instead of P.

The Chase agent claims they cannot change the ticket because they can't go from an all business-class fare to a mixed cabin & business class fare.

They then claimed that if I just cancel the ticket and rebook, I'll get a future flight credit on United that I can basically use for any flight in any fare class as long as it's a United flight (obviously) and same passengers.

Before I cancel this Chase itinerary, I just wanted to double check that that is true? When they say "one leg has to be operated by United", does that mean United metal, or would a United codeshare flight work just as well?