This has been expected for a while with the combination of OD's salary, injury history, and Griffin's performance down the stretch last year. At this point there's still a decent shot that they re-sign Garrett Graham, which may put Griffin second on the depth chart initially, but I fully expect that he'd overtake in camp or shortly thereafter. Graham is a nice chain-mover but Griffin is a much more dynamic player.
There's also been a lot of chatter of the Texans grabbing someone in what looks like a draft full of TE talent so that O'Brien can try to somewhat replicate the TE duo he had in NE. Either way, Griffin is in for some serious snaps this year and IMO, will look like a total steal by the end of the season. Had he been playing in a more competent passing offense, he would have been a much higher round pick when he came out.