I have real concerns about this. Feels like the kind of thing the Cubs are always trying to do, they try to make everybody fit a mold of the few pitching development success stories they have. Justin Steele works because he is a unicorn - it doesn't make sense why he's so good but he is. Cade Horton is a monster - even in Double A we saw 97 with two distinct breaking balls - so why on earth would the Cubs try to mess with that? They're relying on this kid being the ace of the future, but he won't be if they mess with what works, and make a two pitch 93 MPH guy.
In terms of the fastball issue, every scouting report that exists - and a few detailed write ups - talk about the great carry and spin he gets up in the zone. Extension is overrated if you have those other two things, i.e. Justin Verlander. It doesn't make sent to me why the Cubs ALWAYS do this with guys. 93 Cutter is not an ace. 96, 97 fastball with carry, spin, a wipeout slider and slower curveball he can throw for strikes very likely is. Certainly feels like the Cubs are having him sacrifice what made his fastball good in order to fit their very antiquated, weird narrative about what a 4-seam fastball is.