In this American release of the game, the protagonists' names were changed to Jimbo and Sully (descendants of Bill and Lance). This change was most probably made to ensure continuity with the first Contra, where the time line between each version diverged - although the original Japanese game was set in 2631, the American version was inexplicably set in 1987. Interestingly, Konami America decided to finally reconnect with the original futuristic setting, and to set Contra III Alien Wars in 2636. But, in order to remain consistent with the first game, the two protagonists had to become (very) far descendants of Bill and Lance, hence the change.
Since the instruction manuals for the American versions of Contra and Super C for the NES established those games to be set in present times, yet Contra III featured an obvious futuristic setting, Konami explained this discrepancy by claiming that the main characters of Contra III were Jimbo and Sully, the descendants of Bill and Lance.
There are a couple of other notable differences between the original Japanese version and the other releases that are worth noting. First off, Contra Spirits gives players infinite continues, whereas the other releases award the player with a limited amount at the beginning of the game. Then, the Japanese version surprisingly features a couple of cheat codes that were completely omitted from the other releases (such as 'stage select' and 'sound test').