There are a lot more files than that involved.  You really shouldn't mess with it behind the scenes at all; that is why there is a void invoice function.  Since you started, you need to also update the BKARHIVL files, BKGLTRAN (or BKGLTEMP if not yet posted), BKARINVT, BKART, INVTXN, possibly BKPRCOMM or ISREPORD if there are commissions involved, and if it was anything other than a price change, there are inventory implications.