Memberships always run their full monthly course.
Buying a new membership doesn't terminate a current membership prematurely.
The new membership stacks to kick in when your current membership eventually runs its full monthly course and expires.
Sounds like the message that you got may have been a bit misleading?
Can you post the full text of it?
In the meantime unfortunately roaming calls/texts/data will not work because you're in a Don membership and since 13th June EU roaming only works on Guido.
You might want to get onto 48 support and see if they can do anything for you here.
Hope this helps.