It really depends on why you're using a pen name.
A writer I know locally writes in two very different styles. For one, she uses her maiden name (Megan Lindholm); for the other, she uses a pen name (Robin Hobb). She does this because a Megan Lindholm book reads very differently than a Robin Hobb book, and by keeping them under separate names people don't go to read one expecting the other.
Others have used it when they write in different genres. Jim Rigby wrote romance under the pen name Reagan O'Neal, at least one Western under the pen name Jackson O'Reilly, and fantasy novels under the pen name Robert Jordan. However, the fame he gathered as Robert Jordan led the Reagan O'Neal novels to be republished as "by Robert Jordan, writing as Reagan O'Neal" in hopes of garnering sales on the back of the success of Wheel of Time.
Some have used a pen name when trying to separate themselves from the work. There's a series of science fiction adventure novels aimed at younger readers called the Lucky Starr series, which some have cited as a probable partial inspiration for Star Wars. The author, Paul French, was actually Isaac Asimov. The books were originally written to be turned into a television series, and Asimov was concerned the series would turn out badly, so he didn't want his name associated with the books. When the television series was never made, he owned up to being the author; any of the Lucky Starr books you find in print now will read "by Isaac Asimov, writing as Paul French," but the original editions just listed Paul French.
Other people use a pen name because their real name is associated with their work, and they don't want to clutter a Google search. Or because their real name is in use by another author, and they don't want the confusion. Or because their real name is impossible to spell, and they want an easier one to deal with. Etc.
So any answer really comes down to "why do you want to use a pen name now," in terms of whether or not you should republish the first book under a new name.
