Within a reasonable time frame a voter should be allowed to change an "early vote." There could be facts interest that give a voter reason to change their ballot or simply a change of heart. No matter the reason the voter should be granted leave to make a new decision.
Yes, I do believe that early voters should be able to change their votes. In the case of this election, many early voters had put their vote in before the news of the FBI re-opening their investigation into Hillary Clinton. That information many have changed someones vote but at that point it was too late to change.
Believe it or not, many states with early voting allow voters to change their minds -- and their votes -- so long as they do so before Election Day. Thus, Wisconsin, for example, allows a person to change their mind and vote a new ballot up to three times -- with elections officials required to destroy the prior ballots once the new one is received (the last one wins).
Usually if you are doing something early it is because you are sure in what you are doing; just like early enrollment to college. So with that, comes responsibility. If a voter was unsure, they should simply wait. As humans, the mind is always changing. It would be a complicated process if everyone changed their vote.