Your on-property benefits are based on your status at check-in.
It's possible that the rates might change between now and the time of your stay. It could turn out that the with-breakfast rate you have booked today will be less than the without-breakfast rate that you want to book in the future should you achieve status.
If your bookings are at a property without a consistent breakfast benefit (i.e. a full service Hilton or a Doubletree) then you may wish to consult the wiki (
Gold/Diamond Breakfast at Hilton family properties (master thread)) for an idea of what the free breakfast entails. It's possible that the with-breakfast rate includes a full breakfast whereas the "breakfast" for a gold could be a little as a coffee and pastry in a to-go bag, depending on the property.