As part of the "Division 86" reorganization plan, which was planned to be complete by 1986 so most units were converted before that.
It generally happened when units transition from M60A3 tanks to M1 Abrams tanks. As units received the new tanks they transitioned to the new organization. The tank battalions also went from three tank companies to four tank companies during the same process.
But the transition didn't happen all at once, even within units, it usually took a few months to sort out. Some M1 Abrams units maintained the 5-tank platoons for a few months, some M60A3 battalions transitioned to four tank companies before they received Abrams, etc...
Typically a Germany based M60A3 tank battalion went through a 2-3 month process. They cleaned & turned in their old tanks & equipment, deployed to Grafenwoher, received new M1 Abrams & equipment, conducted gunnery training, qualified gunnery, deployed to Hohenfels, conducted a long field training exercise.