this is stupidly simple, i don't know why they have to be 2 separate compiles:

-compile as fullscreen
-change last 2 lines [0,0] in windowed.ini to -1,1 for borderless window, or back to 0's for fullscreen.
-cannot be done while in game, as exiting overwrites it to current mode.

borderless can alt-tab to other apps and back, but the downside on win7 is more tearing due to no vsync.
edit: the downside is true only when desktop composition has been disabled by a classic windows 7 theme, but even with aero theme on, it's vsync is way too erratic compared to win10's .

...actually, since win10 forces a very good vsync on in borderless modes [unless new versions change that] vsync lag is no longer an issue, so only time you even need fullscreen mode is for win7, or if your game can't run at 60fps, which is when vsync should be off [tearing is better than lag/stuttering]