Speaking as someone who enjoys reading the scientific papers coming out of the genetic and microscopic medicine area I can tell you for certain that there are teams in at least ten countries around the world working on things which could one day lead to this happening.
In scientific terms we are surprisingly and tantalizingly close to making it at least theoretically possible, because for example a gene has been discovered that actively and reversibly causes mammalian ovary tissue to change into testes tissue. You activate it and you get one thing, you methylate it and deactivate it and the tissue becomes the reverse, and this has actually been successfully done in laboratory mice. So in theory at least, tissue regeneration into a different form has been demonstrated.
HOWEVER - and it is a whopping great big however, this is just a tiny first step and if you think abut all the other issues involved like for example altering all the internal plumbing and nerve wiring there are huge obstacles still to overcome before this exciting theoretical discovery could become a working gene therapy.
Then of course you will have to overcome all the moral objections form the religious lobbies so while this is exciting it almost certainly wont actually happen within our lifetimes.
Bottom line yes it is already proven as theoretically possible - but no don't hold your breath - because you'll be dead before it actually arrives.
