How will the transition go?
I would agree with the OP that it might take a couple of years for the coaches to get the personnel in place but that could be with every team whether the team is changing schemes or not.
My guess is the Linebackers will have the most difficult challenge to the scheme change. I was always impressed, under Smith's defense, how Urlacher and Co. would always hit the LOS/gaps that, to me, didn't seem like they should be hitting (I was always anticipating them to fly out to the edge, perilously opening up cutback lanes) but actually being extremely disciplined and would only allow a few positive yards on the edge.
Tucker's linebackers were... whatever- no need to discuss.
So for our new 3-4 scheme transition-
DL:
I am not too familiar with the scheme of a 3-4, but the DL, I assume, would be able to coached up, in practice before the season starts. Gap control, Man control, or whatever they do it seems like it would be much easier to simulate in practice. Starters vs. starters. Seems like the new bodies would be fine to run the 3-4.
Secondary:
I wouldn't expect their coverages to be vastly different and nothing that wouldn't be crossover coaching anyway. Everyone runs some man, everyone runs some zone. No team is exclusive to one single coverage.
LBs: I am sure the coaches will rep the assignments 1000 times in practice until the muscles have it in memory. But when the real games start it will be different. Speed, adjustments to moving bodies and gaps and Offensive schemes/personnel will change and this will cause mistakes by our LBs.
That being said, I would expect to see a defense that doesn't break assignments, maybe get outmanned, maybe. I sure am having much more hope than this year. I did not have hope last year strictly due to knowing the defense was going to suck.