It's possible, but very unlikely. Usually it takes a substantial number of genetic differences to qualify two organisms as different species. One might think that reversing the direction of selection pressure could reverse a genetic change, but genetic change begins with random mutations which are _not_ determined by selection pressure. It's very unlikely for one random process to undo another random process (imagine mixing black and white sand in a jar by shaking it, then trying to _unmix_ by _unshaking_!).