Appearance didn't change in any big way. In 1984, all retail cars got 15x7 rims. Colors changed in 1984, 1987, and 1988. Interior colors and material changed in 1984, 1985, and 1988. Radios changed in 1984, and mid-'88. Steering wheels changed in '84, '87, and mid '88. CMHSL began in 1986.
Mechanically:
'83 - E48 heads went from 587's to 345's, all slant sixes were ESC/EFC (US only), last year for 36RH, except taxis (AHV package from '84 up).
'84 - Production moved from Windsor to Fenton. Everything got altitude compensators. This wasn't an emissions requirement; I believe it was more of an attempt to mitigate vapor-lock. The 34RH/A999 was now standard behind the ELE. Some 2bbl police models were built with the 2.94 rear. Last year for Carter carbs (BBD and TQ).
'85 - 2bbl carb was Holley 6280, which was very much a BBD clone. 4bbl got the Quadrajet E4ME (17085433). Some of the EGR and bowl venting parts were updated.
'86 - the 32/34RH received an upgraded TC to reduce shudder. This was also the first time a semi-synthetic fluid was used (7176/Mopar ATF+). New combustion computers were used that helped both timing curves and fuel trims.
'87 - Production moved from Fenton to KAP Main in mid-'87 MY. Stainless exhaust used in all 3 models.
'88 - New alternators (Nip or Mopar) at 90 or 120a. New starters (Nip or Mopar), New starter relay and driver SRS for mid'88.
'89 - Last year for M-body (last day was Dec 21, '88). Most everything had the 8.25 rear, as they were using up remaining inventory. Retail cars still had the 2.26 ratio.