Not the tax, but the charge
The focus on "having to collect the tax" or "waiving the tax" kind of misses the point, I think. It would seem logical that if you change a ticket resulting in a difference in the amount charged, whether tax, fare, or whatever, Delta could decide to not bother to charge amounts less than, say, $1, and also not to credit amounts less than $1.
This would have nothing to do with establishing different fare buckets than are 10 cents less than the old ones, or whatever, it would just be a process at the very end when you are going to charge someone a delta price (shh-boom).
Of course, everyone would b**ch about not getting their 32 cents back.
I'm not saying they have to do that, or even should, but if the cost of running the transaction were over $1, the policy would save them money. It is doubtful that even FT'ers would game the fare system enough to construct scenarios where the change was $0.99 on a consistent basis.
Just my 2 cents worth ...
--- Andy R.