If I switch from my storage plan to family plan what happens to storage from my plan. Does it transfer to the family storage plan

You can continue using your own plan, or you can switch to the family plan pool.

If you switch to the family plan pool, your contents will be preserved, and you may or will receive a credit for the remainder of your now-retiring storage plan.

More details: Share an iCloud storage plan with your family - Apple Support

Depends on you.

When you share a storage plan, your photos and documents stay private and everyone keeps using their own accounts with iCloud—just like when you have your own storage plan. You simply share the space available in the iCloud storage plan with your family members, so you only have one plan to manage.

If you already had an iCloud storage plan

When you share your iCloud storage, any family members using the free 5GB plan are added to your family plan automatically.

If a family member is already paying for their own iCloud storage 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.

Once you’ve switched to a family plan, your individual plan is immediately nullified. And the family plan takes effect.