The mod suggested in the link will not change VCC of the CPU, just the voltage of the analog supply pin and maybe some other points on the board. So the CPU will be outside of the allowed voltages. If you really want to change VCC to 5V, you would have to do a similar mod on J22. But I would not do this before checking if all other parts on the board can take the 5V instead of 3.3V AND there will be no parts left hanging on 3.3V, getting a 5V signal from the CPU. I would not risk to fry my DK for just simply getting a 5V range, a small board with a few OPAs, or one OPA plus some resistors more should do the job.