They started before then with the physical raising of the platforms because the M2 EMU's introduced in the mid-70's didn't have door traps...but some of the Amtrak & diesel platforms stayed low longer. The accessibility project that got all the egresses + platforms compatible with handicap access didn't wrap until the big '85 reno. Which was a big showcase deal stemming from Metro North's '83 founding.Though I'm aware that Union Station was rehabilitated that year, I hadn't realized its platforms were low till then. (IIRC, platforms at the other stations along the New Haven line were raised in the early 1970s.)
NH Union spent a solid 10 years in total configuration disarray because MNRR's public spinoff was so much-delayed compared to all other Northeastern commuter rail agencies. Nothing was getting done in the decade of rudderless "Conrail Metro Region featuring ConnDOT fun bux" ops, so the mid-80's were a big burst of state-of-repair renewal flushed through after somebody finally got put in charge.