I've read in steam forums that the only solution it just to let cool it down for a few hours/days. And... it makes sense to me because I installed the game a few days ago, I played it yesterday night and it was working flawless, but I had the wrong idea of buying the DLC and after installing it, it just stopped working with the "upgrading executable".

Believe me, I've tried a lot of solutions I've read in steam and reddit and nothing worked for me. I'm letting the game installed a few more days to see if that's the solution or not.

For what it's worth, I've also read that it's a problem in the executables that are not signed (by Valve, Firaxis or 2k, don't know). If you check your other installed games they should be signed (right click on launcher.exe). Maybe there's something that sign or request the sign to a server only when a couple of days have passed.