Had this problem, even tried manually downloading 7to8 but still received "source file checksum error".
Solution: I had edited 2 of the .ini files to disable bSmoothFrameRate and changed the FOV in another. I had set both .ini files to "Read-Only" so they wouldn't reset when I launched the game. All I did was return the original files (no longer read-only) and it was able to update no problem.
I'm guessing this problem can happen for other reasons, but if you made any file "read-only", I'd just undo it so it can update your files without this error.
Solution: I had edited 2 of the .ini files to disable bSmoothFrameRate and changed the FOV in another. I had set both .ini files to "Read-Only" so they wouldn't reset when I launched the game. All I did was return the original files (no longer read-only) and it was able to update no problem.
I'm guessing this problem can happen for other reasons, but if you made any file "read-only", I'd just undo it so it can update your files without this error.