Good Day! Searched the forums but couldn't find the answer. Is the only way to book a mixed cabin flight to make the initial reservation with DL.com and then call in to DL to change the desired leg to a different fare class?

For example, I wanted to book ROC-DTW-LAX okay with Y or C+ for ROC-ATL but wanted F for ATL-SEA. Are there advantages to have the receipt say it's an F fare outright vs doing the buy up and having the e-ticket be Y or C+ with an add-on?

Thanks in advance!