(1) “Instant change”? Are you saying that one morning everybody woke up and they had all changed to the same skin color? Seriously?
(2) How do you know this?
(3) Are you saying that it was a miraculous change, that God simply ordered all DNA to change simultaneously to start expressing the already-existing alleles?
If it was such a Divine miracle, then we are not in the realm of science and there’s not much to discuss. (Of course, if you are claiming that this is Bible theology issue, then you can cite the Bible passages which form the basis of your claim.)
How do you know that the mutations were already within the code/blueprint?
If you can tell us where the mutations are stored (and how), then scientists could verify your claim.