You were arrested for 2nd degree controlled substance the person whos it actually was wants to come forward and take those charges what will happen?

They wont just swap the charge just like that. the case and story has to match. If they found it on YOU the YOU will be charged PERIOD. And if they find out that you both are making it up both you and him are getting charged with abstruction of justice and he will also most likely get a interfearing in an ongoing investigation followed by making a false statement. If you gonna make em lie for you trust me it ain't worth it cause if it backfires it will backfire hard. good luck