I can’t understand your meaning. I suggest that you read the fare rules carefully several times.

Regardless, the phone agent is right. You cannot upgrade or downgrade fare class without in effect canceling the ticket and buying a new one at the current fare. As you approach the day of travel the WGA fare will get quite high.

You need to decide between WGA+ and WGA at the time of purchase or at least 2 weeks before the trip, whichever is later. Otherwise the cost makes changing the fare class prohibitive.