Originally Posted by
Smirnoff
You don't need the underlying fare class to be available, only the award (upgrade class).
At long as there's U class available, you can change (on the phone), whether there is T class available or not, subject to the change rules/fees of the underlying T fare basis.
Sure - but if T is available, then the cost to change is just the change fee. If it's not, then you're looking at changing from T to E or W - which is much more expensive.