According to Marg Helgenberger when interviewed by French magazines, TV Mag and Télé Star recently, yes he will.

I don’t want a teary goodbye. I want it to be dramatic and mysterious. All I can tell you is that William Petersen promised to return so Grissom will be there for Catherine when she leaves. He’ll be there in the first episodes.

In case you haven’t seen the articles or missed our re-tweets, you can read the articles at  Margamania.net

Please note, as also stated on Margamania, this is not “set in stone” as a lot can happen between now and then.

It’s too early for us to add anything further at this time.