Every time the Cowboys switch DC's that wasn't in-house, the defense takes a nosedive for a few seasons.
Some players, even Pro Bowlers will have a problem adjusting to a new style of play. Some will not be suitable for the new responsibilities or role. They will have to be replaced.
For example, when Wade Phillips took over as both HC and DC for the Cowboys, he did run a 3-4 defense like Zimmer did with Parcells. However, the similarities stop there.
Under Parcells, Zimmer used a traditional 3-4 defense. The front three were big guys whose job was to hold the line and fill the gaps so the four LB's could search and destroy. The DE's were close to being almost DT's The Cowboys used Spears, Canty, Ratliff, and Hatcher. Occasionally, Ratliff was used as a NT to relieve Ferguson.
Ferguson was a huge wide body whose job was mostly to get in the way and force RB's to take a detour.
The pass rush was predicated on the "mystery" pass rusher. The front three would rush but the 4th pass rusher could be from anywhere. It could be a LB. It could be a safety. It could even be a CB. When they blitzed, we are talking about two defenders coming out of nowhere. This confused offenses which made the pass rush more effective.
In Phillip's 3-4, the DE's were more dynamic., They rushed the passer and did not lay back for possible running plays. In order to achieve this, Phillips got rid of Canty and Ferguson and replaced them with Olshansky and Ratliff. Ratliff played great against the pass but was a liability against the run.
Another element of Phillip's 3-4 was that DE Demarcus Ware became almost an automatic pass rusher. Offenses knew he was coming in, there was no mystery. In effect, their 3-4 was actually a 4-3 out of position.
I'm wondering how our two All Pro CB's are going to react to less man to man coverage under Zimmer. How does he intend to use Parsons and what will be the learning curve there? How long will it take for him to get the type of LB's he needs for his defense and what do they do in the meantime?
The issue here is that the Cowboys urgently need to upgrade their running game with at least one, probably two draft picks going to RB's and others going to the O-line. In effect, both the offense and defense will be forced to transition at the same time. That's not good.