Hogwarts' Ghosts

[Jimmy and Tamee Livingston]

No: HPFGUIDX 51546

There are a lot of things I want to know about the Hogwarts' ghosts and the recent speculation has brought up most of them (I stupidly deleted the relevant messages so I can't quote from them).  On thing though, that keeps bothering is how the House ghosts get that position and how long has the position been around.  I mean NHN, the Griffindor ghost has been dead 500 years at the time of COS and Hogwarts is a 1000 years old more or less, so he can't possibly have been the Griffindor ghost since the beginning.  Is he just one in a series of ghosts who's had the position?  If so, is this an indication that ghosts can move on from the earthly plane and that there is a chance that Binns could move on leaving a vacancy for a new history teacher?

Or did Hogwarts not have any serious ghostly presence for several hundred years?  The only ghost we know definitely died on Hogwarts' grounds is Myrtle.  Were many of the ghosts gathered there during the period that the WW was shutting itself off from the MW and working on hiding magical creatures from them as well?  Were some of the ghosts "exorcised" from their original dwellings and came to Hogwarts as a refuge?  

These are some of the things I want to know and am seriously hoping that OoP will answer at least a few, maybe if nothing else what the deal is with the Bloody Baron.  I hope I didn't just copy some speculation from anyone else.  If so, I'm red with embarrassment and apologize.

Tamee