Every decade brings changes and I think Elvis would have gone through these changes as well. There could have been a chance he would dress himself in the two-piece style he used during 1975 or maybe in the style Neil Diamond used with a little resemblance to the earlier years.
His hair would also go through a change although I do not believe he would cut his sideburns before he would lose some weight. As Elvis face become more swollen he also grew more sideburns to hide a little face and made it look thinner. Maybe he would stop dye his hair and allow a little greyness.
But...it's pure speculation. Remember that while everyone cut his sideburns Bruce Springsteen developped them ore and more in Elvis way.