Sorry Magic Planeswalkers; the card unnatural selection changes the creature type on the creature targetted.  Just because the creature type becomes flagbearer it does not take on the ability of flagbearer which is clearly defined on the three flagbearer cards from Apocolypse.  There is no situation in which changing a creature to a flagbearer will prove to be of great benefit except when a coat of arms is in play.  If your opponent controls a flagbearer then you have to target the flagbearer and not your own creature.  You can make your opponents flagbearer into a rebel or some other creature type and then after resolution can change your creature to a flagbearer type without the abilities of flagbearer that are defined on the flagbearer cards.  I don't believe that an opponent's Ursa's Rage that has been kickered could be made to target your own and never on your opponents flagbearer by name only or an actual; flagbearer with the abilties listed on the the card.  If a player controls a flagbearer with flagbearer ablilities appearing on the card, he or she does not have to target spells they cast on his or her own flagbearers.
Unnatural Selection is still a powerful sideboard card in mirror matchups, and is seen in great prevalence at this years world championships in type 2 format.  It does great in taking down an army of saprolings in both fireburst as well as sapropling opposition decks.
Back to flagbearers and unnatural selection, there is still some utility to the card in this area changing your opponents flagbearer into some other creature type allowing you to target oneof your or his other creatures.
Wayne aka