Fee for this change within 21 days?

Searched and could not find answer for this specific situation.

No status on United. I have an F award ticket booked Phoenix-LAX-Seoul-Phnom Penh. The last leg is currently in economy :( I have an expertflyer notification set up in case business space opens up on the last leg.

My question: if biz opens up within 21 days of departure and I call to change it, will that incur a $75 change fee? I'm just changing to an award I paid for.

If so, I'll cancel my expertflyer subscription since that is all I use it for.

Thanks.