If they could make Jeff Bridges younger for Tron Legacy, aging Ewan to Alec Guinness' age in Episode IV ought not to be a problem. It might even be possible to resurrect Alec the way Marlon Brando was brought back for Superman Returns....
The young Jeff Bridges looked weird in Tron Legacy. Not bad, just not quite right.
However, after watching the work that was done with Chris Evans in Captain America, I think they could make anything work.