You are assuming there is a published S class fare for your new route.... and that you meet the advance purchase requirements to access said fare. Why would your TA assume you are ready to pay a massive adcol for your new fare plus the penalty
If you didn't say, make sure I stay in tango plus, its your fault. sorry.
It's exactly the same orig and destination, on the same fare code with plenty of availabilty on the new route. Small refund of taxes, slightly larger increase in other costs which may be just currency fluctuation.
I don't understand why the TA would rebook into lowest available bucket, leaving 20% of the original cash paid with AC but used for nothing.
No one is explaining why that makes sense, although from the comments so far ir appears to be what people would expect.
Lesson learned I guess.