If you already have a myCloud account, your limited (15GB) or unlimited myCloud storage will be converted into the corresponding new subscription. Which subscription your account will be converted into can be seen in the informative email we already sent you or you have yet to receive.
Please note: should your current storage usage be higher than your storage space available after switching, you might experience limitations when using your account. You can read more about this here.
You can find more detailed information about the converting options on the following help pages:
What happens if I currently have an unlimited myCloud account?
What happens if I currently have a myCloud account with 15GB of storage space?