How to downgrade GTA IV Complete Edition to EFLC

So you've got latest GTA IV Complete Edition on Steam/Rockstar Launcher or Fitgirl repack and you want to turn it into original binary-identical EFLC (Episodes From Liberty City). This downpatch made in ISXPM in conjunction with official Rockstar EFLC Patch 2 or 3 (both included in the package) does just that.