The old trick of adjusting the year via edits of the Options.ini and SingleMission ini from 1st Gens (SF/SFG/WoV/WoE/WoI) no longer works, as those statements have been removed from those inis.
Ya see, TK went and changed how things work on us, and it probably pertains to 'fixing' aircraft and object types to specific terrains and time frames and the 'LimitedNations' statements ... instead of having it listed in the singlemission or options inis for global game use, it now contained in EACH TERRAIN'S MAIN INI. Probably a 'merged' useage thingy.
Important Note: read this ALL the way thorugh before attempting the adjustments!!
First off, you'll need Gerwin's Cat Extractor*. Unfortunately, the thing will unpack ALL 100-odd megs of stuff from the terrain cat, when you only need the singele ***.ini (ie: Desert.ini, GermanyCE.ini, VietnamSEA.ini, IsrealME.ini).
Follow the instructions for use on Gerwin's Extractor.
Keep said main ini, and THROW OUT EVERYTHING ELSE. (however, you may want to keep the types, targets, movements ini in the extracted folder. As a 'just in case I want to mod this terrain' kinda thing
Using the VietnamSEA.ini as an example (all the others WILL be similiar), this is the entire VietnamSEA.ini, you'll note the relevant statements at the bottom, higlighted in boldface: