He won't be the same, but providing he recognises and embraces the changes he will still be able to perform at the highest level. Ryan Giggs is a good example. In his youth he was a fleet footed pacey winger - the type that England have desperately needed for his entire career. As he has aged he has developed into a more rounded attacking midfielder and won Footballer of the Year recently.
The new no refuelling rule change will benefit Schumi in this respect. For the past years a Grand Prix has actually been 2, 3 or 4 sprints glued together by pitstops. Not any more. A more considered approach to tyre management, like we had in the early 80s, will be to his advantage.
Also, although not as old, Lauda took a lengthy break, came back and was competitive.