I wouldn't call that event a major change because (besides that one detail) history still turned out relatively the same.
In other words: Sisko (and we as an audience) would have been the first to notice any other major changes within the time line when Sisko replaced Gabriel Bell. So obviously it didn't matter who Gabriel Bell was. Just as long as someone played out his role in that particular point in history for it to turn out relatively the same (as we know it).
Also, in point of fact - the only episode that shows a major event change is Shockwave where Daniels informs Archer that the destryed colony incident NEVER happened. While there was a lot more time travel episodes in ENT's 4 seasons, there wasn't any more alterration of history then in previous Star Trek time travel episodes dating all the way back to City on The Edge of Forever. Please show me one major Star Trek timeline event change that occured becauuse of ENT's time travel episodes that compare with either the DS9 episode listed above, or hell, the timeline change in First Contact the film - which have 24th century characters being a part of Zefrim Chochrane's first warp flight?
I've already written a post on Star Trek's time episodes in comparison to Enterprise. You can check it out here...
http://trekbbs.com/showpost.php?p=4009267&postcount=110
However, to add to the argument I already presented: The Xindi Attack was brought about by the Sphere Builders who were apart of the Temporal Cold War. And the Xindi Incident was not erased from the time line but continued to exist (as shown to us in ENT's "Home"). This suggests that a major incident (that wasn't supposed to have happened) effected the time line. Which means we are probably looking at an alternate time line and not the prime time line.
In other words: if there are only minor changes or deviations in the time line, I consider it to be a part of the prime time line because there are no huge differences within the time line. If there are major influences or changes in the time line as presented to us within Enterprise: I consider that to be an alternate time line.