That was pretty much my understanding too.
The USMC acquired it's M1s/M1A1's between Nov 1989 (211 MBTs) and 2008 (44 MBTs used to increase the size of the two active duty tank battalions by one tank company each plus some for maintenance float). By FY 2009 the entire fleet of USMC M1A1s had received the FEP upgrades (US military Fiscal Years are the year prior to the date, thus FY 2009 is calendar year 2008).
The FEP was an improvement to the fire control and vision systems not armor, stabilization, or survivibility.
Starting around 2013 all USMC M1A1 FEPs began rotating thru the rebuild cycle at Anniston Army Depot, and an unknown (but fairly small) number were upgraded from M1A1 to M1A1 Heavy Common (thus more armor). Around 2015 M1A1s and LAVs started receiving improved thermal sights. Trophy mountings/electronics etc. were added around 2016. And AIDATS (improvements to the commanders .50cal) around 2018.
So a change from 2008 to 2009 for the FEP would seem reasonable. And FASTBOAT undoubtedly knows FAR more about vision and fire control systems then I do so his figures of TI/GSR 50 and FC 60 should be considered accurate.
But as with many units in WinSPMBT ... how many incremental upgrades are required before a new unit needs to be introduced? Does it make more sense to have a unit that starts before a specific upgrade is fielded incorporate that upgrade then wait several years before cumulative upgrades require the introduction of a new unit?
https://apps.dtic.mil/dtic/tr/fulltext/u2/a608067.pdf