Pfft. Easy. All you need to do is dowmload a nifty tool call GH3S, found right
here. When you open it, open your ROM. Then just select the Pokémon, you want to edit.
Since we are talking evolution, goto Evolutions, after you select the Pokémon. Click on the evolution you want to replace, just remeber that we are changing the NFE, not the evolved form, so evolution to Machamp is on Machoke. Got it? Okay good.
So after it is selected, goto the bottom, and click on setting on how it shoukd evolve. So the first option you would click "Level", then on the second one, click "32", on the last one, click whoever you want it to evolve into, in our case Machamp. Then click Replace Evolution and click Save Tab. THEN BAM ITS DONE. Easy amirite?
Make sure to save the ROM afterwards, and always back it up, in case something goes wrong. Super simple. Hope this helps, and future luck to your YouTube Channel.