I might be misremembering but I didn't think they brought in Martin Sheen as a late addition. They always wanted the president to be in it and to be played by a big name actor (several were considered - I remember reading that Sidney Poitier and Alan Alda were others) but the original idea was that he'd be a distant figure, included as a recurring character who might only appear every few episodes. Sheen's casting as Bartlet wasn't inconsistent with Sam being the central character. The show was meant to be about the White House staffers and the way Bartlet was treated in the pilot (talked about a lot but only appearing in that one scene at the end) was meant to be the norm.

The change was the decision to then promote Bartlet to a main character who appeared in every (or nearly every) episode and effectively become the nearest thing to a 'main' character for much of the its run.