Pretty fitting for the bears to go to an outdated defense...
Trevathan and Roquan both can't play Will, the position both would be best at. True Mikes don't really come around anymore, ever, so square peg in a round hole time. You'd need a Sam and that isn't anybody on the roster presently.
The secondary scheme would change, not for the better. Fuller especially, and JJ coming out of Utah/his rookie season are press/man guys, not that playing zone (likely cover 2) is impossible to learn but I recall Nnamdi Asomugah and how he went from a shutdown guy in a 34 in Oakland to incompetent/useless in Philly because he couldn't play 4-3 Cover 2 zone to save his life.
Biggest downside (or upside for our board's armchair GMs with a high draft pick) would be Erin Rodgers would gut this team as he did under Lovie and Tucker. Can't wait for opposing WRs to quick slant this scheme into submission with 3 yard quick passes that turn into 20+ because Danny Trevathan is the target to pick on.
Wade Phillips is 73 going on 74 in a couple of months. I'd try to throw as much $ as I could at him for the short term and stay with the scheme you actually have the personnel for...but why do the smart thing?