The morphine will make a big difference, but no surprise your doctor isn’t exactly right about the dose conversion. The equanalgesic dose of oral morphine to methadone is 30mg morphine to 5-10mg of methadone. So a more appropriate dose of morphine to replace 8mg methadone, you’d need 24-32mg of morphine.
Now, 15mg morphine isn’t far off from that all things considered, so it shouldn’t be too painful. If you can continue tapering your methadone before beginning the morphine @15mg, it would be best. But you should be able to manage the transition right now without terrible discomfort.
Other than the dosage issue, morphine is a great replacement for methadone. Is this going to be an ongoing treatment or something you’re going to be prescribe just short term to get off methadone? Also, is it an extended release formulation of morphine or instant release?