how do i change the family organizer without losing our purchases

If all the same family member are in the new family, you will retain access to all the purchases.

You need to disband the existing family using the steps in the article below. The new organizer can then set up the new family and invite the family members to join the new family. If there are children under 13 years old, the new organizer needs to leave the existing family, set up the new family, and then the under 13 years old children can be transferred to the new family. Then disband the old family and join the new family.

Family Sharing -  Transfer a child to a different family group - Apple Support   *

Family Sharing - Leave.  *