Welcome to HW! This question is really best answered by a pharmacist. Call your pharmacy for the best answer.
Based on this conversion guide:champ.bsd.uchicago.edu/documents/Pallpaincard2009update.pdf
120mg morphine cr is exactly equal to 80mg Oxycontin (oxycodone cr). However, conversion charts are not always accurate. Equigesic dosing is typically determined in ranges. The charts usually rely on the middle of the range.
Plus, you are likely to have some overlap when you first change over. If you are having any issues with the morphine being too strong or not strong enough, call your doctor.