The changes we know are mainly minor, what we know;
-All the ResidentEvil.Net stuff (costumes for Mercs modes) are included in the game itself now. These would be the alt colors for the the unlockable costumes, and the retro "PS1 Graphic" costumes for all the characters.
-All of the DLC is included, so a total of 10 Mercenaries stages, and the 3 multiplayer modes, Onslaught, Siege, and Predator.
-There is new and easier online lobbying to help co-op games, include the extra modes in regular Mercs searches, and be able to easily join other people's games who are at cross-campaign intersections.
-There is a new control scheme that will be the default scheme, which hasn't been detailed but apparently is supposed to make play a lot easier. They also have mentioned all of the vehicle sections have new control schemes to make them easier.
-The game's default settings have apparently been tinkered with for some better defaults (no specifics, but likely changed the default FOV, maybe turned QTEs off/laser sights on by default, as a possibility).
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We don't know anything else than that, but it sounds like while it isn't strictly the PC version, the changes are mostly minor to our knowledge.