Yes, people are absolutely allowed to change. But if between the ages of 46 and 49 you transform from a person who despises and badmouths the most outspoken libertarian/Constitutionalist in America into a person who calls himself a Libertarian/Constitutionalist who supports nearly every position the person you just disrespected supported, then you are going to have to expect some damage to your credibility.
And no, I wasn't born a Libertarian. I became one in college when I started following politics. Perhaps I still have some evolving of myself to do. However, Glenn Beck has been studying and following history and politics for decades and in the last year he changes from hating Libertarians to becoming one? People have a right to question this guys' motives. And his motives are, of course, to boost ratings, expand his business, and make money. If he helps the country, that is an added benefit.
"Where liberty is, there is my country." -Benjamin Franklin