Well, of course such a population is still classified as fruit flies but that's due to the fact that they descended from the "fruit fly node".
And for the same reason we are still primates and vertebrates.
Some things are irreversible. Breaking glass. burning books. Long stretches of evolution.
Once a series of mutations has happened in a certain order, it is unlikely that they will ever happen in exactly the reverse order.
Before a mutation happens there is no requirement for it and no specification. But if you are trying to reverse evolution experimentally, then you have a specification, all the probability calculations that the ID crowd is fond of are applicable.
For this and other reasons, it is unlikely that there will ever be radically new body plans evolving out of established species. New body plans can only emerge from rather simple body plans.