Gregg wanted to give the good news to all his fans so they wouldn't worry about whether or not he'd be in the Wizards reunion movie:
“I’m sure you guys will be happy/excited to know that I have officially signed onto to play the one and only Mr. Mason Greyback again. Wizards Re-Union Movie! Get ready! You guys happy I’m officially back?”
This is great news, especially after fans found out that David Henrie won't be appearing in the reunion movie.
Source: Disney Dreaming