I was on methadone for over sixteen years and stopped march of this year and chang to suboxone. i still test positive for methadone. how is this po?

Not unusual: I have seen people who were on higher doses of methadone, as well as people with very long methadone half-lives, stay positive for up to 6 months. Methadone is stored in the liver and other tissues, and is slowly released. If levels are done (corrected for urine concentration) the levels of methadone itself should be going down and already be very low, with gradually lower levels of metabolite.