I'm a little surprised at the idea of Denver and KC switching to the 3-4. I understand that Pioli has mainly scouted 3-4 players, but his skill has been more finding players to fit a system than exclusively being a 3-4 guy. McDaniels comes from an offensive background, so it shouldn't matter to him. Mike Nolan has a 3-4 background, but I also believe some 4-3 mixed in as well.
It seems to me that switching schemes when the personnel aren't well adapted to it is a potentially big mistake. The Jets are a good example of how hard the change can be. I understand that Denver has a lot of holes and that some of their recent picks (e.g., Jarvis Moss) might be adapted to a 3-4 switch, but others less so. DJ Williams is probably the best player currently on their front 7, and he is 6-1 242# - not really the site (or temperment) to play 3-4 OLB. They could try moving him back inside to 3-4 WILB, but they have already spent a lot of time moving him around from ILB to OLB, which can really mess a player up. Jamie Winborn is 5-11" 230#, and Wesley Woodyard, whom they converted from S to OLB last year, is 6' 230# - more the speedy kind of OLBs for a Cover 2 scheme than the big OLBs suited to take on blockers in a 3-4 scheme. Tim Crower at 6'4" 275# is too small for an effective 3-4 DE. I could see Moss moving to 3-4 OLB and DJ Williams moving back inside, but it seems like they would have to rebuild their whole defense from scratch.
KC is even more perplexing to me. Glenn Dorsey at DT and Derrick Johnson at OLB are just not at all physically suited to the demands of a 3-4 defense. In addition, it is probably harder to find suitable 3-4 personnel at the DT, DE, and LB positions, and may take years to develop or convert, so it would seem to make a rebuilding job harder.
Mike Tomlin took over the Pittsburgh job coming from a 4-3 DC background, and there was speculation about whether he would try to convert Pittsburgh to a 4-3 team. He wisely left **** LeBeau and the defense alone, continuing to draft players such as Lamar Woodley who fit their existing scheme. I understand that KC and Denver are in much more of a rebuilding mode than Pittsburgh, but it still seems to me like this is not necessarily a recipe for success.