Early upgrades can happen any time upgrade inventory is available, so if RM releases inventory for any reason, apparently upgrading can occur.
My upgrade on AA 72 SYD-LAX 23 Dec 2015 cleared prior to the 100 hour window.
Our (2) upgrades for AA 164 SFO-JFK on 6 Jan and return AA 17 on 16 Jan cleared well before T-100.
AA 80 DFW-LHR 13 Apr, and AA 109 LHR-LAX 18 Jul have already cleared.
All J cabin to F cabin, all but AA 80 with SWU; AA 80 with BXP3.