Just because there is original fare class availability on new flights does not mean there is no potential fare difference. You are not considering advance purchase requirements on cheaper fares. If you are not changing same day, you will generally be subject to AP requirements based on ticket re-issue date, not original purchase date. Original booking class availability on flights let you SDC without regard to advance purchase and potential other fare rule requirements (certain fares can also have time of day requirements). Here are part of fare rules for DTW-LAX XFVQJ0MF roundtrip fares with 21-day AP and time of day restrictions. You would normally not be allowed to keep these fares if you are re-booking flights less than 21 days out or your new flights do not meet day/time requirements of originally booked flights even if there is X bucket available on new flights. These fares also have a non-stop routing requirement (online SDC will require following fare routing rules, while agents can at times waive them).