I hate that she's leaving, too. Though Emily is my favorite, the show won't be the same without AJ/JJ.
They haven't actually said anything as to how she's going to leave. The woman who runs the criminalmindsblog ( http://criminalmindsfanatic.blogspot.com/ ) said that she's read the script and that (in her mind) it does JJ justice. Whatever that means.
The episode she leaves is the second of the upcoming season, titled "JJ". The synopsis goes: While JJ attempts to reunite a family whose daughter has mysteriously disappeared, her BAU family braces for the loss of one of their own.
As for as for replacing her, the synopsis for episode three goes: The BAU team searches for a serial killer who is preying on married couples. Also, Hotchner looks to one of the team to fill JJ's vacant role.
So for now, there will be no new characters introduced. But apparently that'll change when Emily leaves.
memory, spoilertv.com and http://criminalmindsfanatic.blogspot.com/