You can't just change the G90 to G91. The code won't run.

And no, that's not how it works. When your CAM software creates code in Absolute format, it's in absolute positions from where you zero'd it.

When g-code is in Incremental mode (G91), the position of each line of code is relative to the previous line of code.

You're post processor adds the G90 to make sure that it is run in Absolute mode.