Originally Posted by ijgordon

"If the itinerary has more than one fare rule, the most restrictive rule applies to the entire itinerary."
So you can't really do what you're trying to (and clearly you're not the first person to think of it ).
Just book the T fare and pay the change fee after you finalize your plans. There will not be a fare difference, unless there is no "T" inventory on your desired return flight, then you'll have to buy up to whatever is available.

Alternatively, if you're traveling in a market where one-way fares are available at half the round-trips, then you could purchase two separate one-way fares. However, there's still really no benefit of booking a one-way B fare on the return trip that you will likely have to change. You'll just end up having to cancel that fare and rebook at the lowest available fare at that time (which may be more or less expensive than the current T fare).