1. Promoting within allows OU to keep things similar—This is key because it figures to keep the offensive philosophies the same. There won't be a major shift in the types of formations they run out of and plays they use from those formations. In the last few years, Wilson has really excelled in calling plays, exemplified by the prolific offenses he's led. And moving Heupel up from QB coach to the play caller and using Norvell in game planning is certain to allow the Sooners to keep it as close as possible to what Wilson has established, at least schematically.
2. Heupel is adept at OU's system—Suffice it to say he's been around the OU offense long enough and gained enough experience to take over. Obviously, he was the quarterback when OU won the national championship back in 2000. And he's since coached three different quarterbacks to Big 12 Championships, including Paul Thompson, Sam Bradford and now Landry Jones. Heupel has enough knowledge of the game and experience to take over the reins at play calling.
3. Norvell has experience as offensive coordinator—From 2004-06, he was Nebraska's offensive coordinator under head coach Bill Callahan, although Callahan was the primary play-caller. He gained experience there, as well as more recently when he was the offensive coordinator at UCLA. So, it's not like Norvell's in the dark as far as what to do and what to expect.
Norvell's experience in the position, coupled with the fact that Heupel has been in the system for years, and they're both from within, are all reasons to believe Stoops made the right decision in moving these two up to operate as co-offensive coordinators.