If i switch from my personal iCloud storage to a family plan, do i have to cancel my personal plan?
If I join a family iCloud plan, do I need to cancel my personal plan , or does it automatically cancel?
It is automatic.
For information about switching to a family storage plan, see: Share iCloud+ with your family - Apple Support including the following excerpt:
If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.