Change from Gold Roger to Gol D. Roger Was there ever a reason given for the name change? I thought at first that it was possibly a mistranslation in the initial episodes. However, in LogueTown Luffy visits a bar named "Gold Roger" and considering the owner knew Gold Roger you'd think they'd get the name right.
There hasn't been any change, I think. 

The real and first name is ゴール・D・ ロジャー (Gōru D. Rojā, Gol D. Roger), but he is also known by the name of Gold Roger. I think it's just an in-universe thing.

That said, his real name carries the D. which in the story is being described as "more than a simple initial", often mentioned when talking about The Will of the D:


  In the series, few characters have the middle initial "D.". This mysterious middle initial has been called "the Will of D" (Dの意志 - D no Ishi) or "D's will" by several characters in One Piece. [...]


By the way, the  meaning for the D. has not been revealed yet, but it certainly holds great importance and is likely to be a major point in the development of the One Piece story.