I thought the rule was if the fare went down a voucher could be requested. If the fare goes down over the change fee amount a credit back to the original purchase could be received minus the change fee or a voucher for the entire difference.

If the rule is quoted as above the airline is doing nothing; they really do not have to. You can always use an unsed ticket for payment of another ticket. In this case you are just rebuying your own ticket routing.