DL agents are notoriously bad at the SDC policy. W fare SDC rules read like a first class ticket, you can change as long as a seat is available and you follow the SDC routing rules. You need to read the rules from the website to them verbatim and ask for a supervisor if they push back. If there are no W seats available then it's possible that the Main Cabin SDC will then rely on the underlying fare class (I'm unsure on this b/c I've never dealt with this particular scenario before).

Here is the SDC rule for W fare from the website:
"If you are traveling in Delta Comfort+ on a W fare, you may make a same-day confirmed change as long as a seat is available in Delta Comfort+. If there is not a seat available in Delta Comfort+, you may be eligible to Same Day Confirm in a Main Cabin seat in some circumstances."