If you're just talking about moving from Tango to Flex with no other changes, AC should do that over the phone with no change fee, just the fare difference.

If you want to re-route, even if it's just changing connections and leaving the O/D the same, that may open a whole can of worms. If you change the airport, it may lead to substantial repricing.

But there's no harm in calling AC and asking what it would cost. I'd be that on a Tango fare it's going to at least incur a change fee though.