William the Conqueror came over and conquered England, becoming king instead of the Anglo-Saxon guy he conquered. That much I know.
And in short order, all the nobility in England were Norman French. That much I think I know.
What I'm wondering about is, what happened to the old bosses? Were they just kicked out of their estates? Killed? Forced to let French people inherit? Something else? I don't really know what the possibilities are here...
A sizeable number were killed in the battles of Stanford Bridge and Hastings so they would have been straightaway replaced by norman barons. After that many were exiled or fled or simple became members of the peasantry and were thus replaced. Some held on a lot longer through, especially further north (though several must have died in the Harrying of the North). By 1087 only 8% of England was owned by Anglo-Saxon Lords, so the rest must have been kicked out, run away, or killed (many joined rebellions that were subsequently beaten by William, and so several probably died in these as well)
Hope that helped