Well, the midi fader only appears in Midi tracks, not Instrument tracks. So if you had those in the earlier projects they would still be there.
You said
Which I understood as: I have Instrument Tracks that’d need to be converted to rack ones. That’s what I’m stuck on.
But it sounds like you want to have midi faders on all your tracks, so: use midi channels routed to either Instrument Tracks or Rack Instruments.