Believe it or not, but Tom Paris was never supposed to be in the series. Robert Duncan McNeill was always supposed to be, but it was never supposed to be as the initially aggressive and self-destructive Paris. Instead, McNeill was supposed to play Nicholas Locarno, a character created by Star Trek: The Next Generation as a character with possible spinoff potential. Not on his own, of course, but as part of Star Trek’s then-potential third show.
That’s why if you ever watch the TNG and Voyager, Locarno and Paris are basically character copies of one another. They’re both roguish, they’re both charmers, they’re both trouble makers and they’re both played by McNeill. Though that last one is kind of obvious.
Locarno debuted in the 19th episode of the fifth season, “the First Duty”, as a member of Nova Squadron; a group of Starfleet’s best cadets. After a crash in a shuttle kills a crewmate, Locarno tries to cover up how he died.
According to Voyager Producer Jeri Taylor, she was under the assumption from the beginning that Locarno was going to be one of the main protagonists of the show. As told in A Vision of the Future – Star Trek: Voyager it wasn’t until the series bible came out that anyone knew the character was changed. This may have been due to rights issues over the character, though that’s never been confirmed. The “official” story, however, was that the series creators thought Locarno was “irredeemable”.
TNG writers Ronald Moore and Naren Shankar didn’t believe that at all, saying as much in the TNG Season 5 Blu-ray audio commentary for the episode “The First Duty”.
The name change and subsequent denial of the character supposed to be one in the same may have been a legal maneuver by Paramount to get the character in without having to deal with the legality of it all. They even used a photo of McNeill as Locarno for a stand-in as a young Paris. The photo can be found onAdmiral Paris’ desk in Pathfinder.