There are a lot of questions going around personnel-wise about the scheme change to the 3-4, but the most popular question is what will Horton do with Jurrell Casey? However, that might not be as big of a question as most people think.
While the Titans are expected to shift to a base 3-4 scheme, there will be a lot of 4-3 packages where Casey will play his familiar DT role. Horton is a very smart defensive coordinator that molds his defense to exploit the weak points in opposing offenses, and I doubt he would waste a player of Jurrell Casey's caliber.
I expect to see Casey playing 5-tech in base packages given his ability to anchor and hold a blocker, and then shift back into a 3-tech DT when the Titans show their 4-3 look.
History of success
Don't forget, that during his two year stint as DC in Arizona he put defensive linemen in great positions to make plays. For example, in his two years as coordinator 5-tech Calais Campbell had 14.5 sacks. Now, Campbell is a different type of player than Casey, but that doesn't mean that Horton won't be able to replicate success.
Even if Horton can't get Casey to double-digit sacks again, that isn't the worst thing in the world. A strong interior pass rush is great, but Horton manufactures interior pressure with smart blitz schemes. This means a speedy linebacker like Zach Brown may be the new face of the defense given his athletic ability and history with getting to the quarterback when he is asked too.
The point is that, while I expect Casey to be successful in the new scheme, the defense as a whole needs to succeed. While players like Casey and Morgan may find themselves in a reduced role, players like Brown, Akeem Ayers, Ropati Pitoitua, and Kamerion Wimbley could all see a major increase in production.