Here's how to transfer a tree from one organization account to another:

If you have access to trees in both the SOURCE and DESTINATION organizations:

1. Open the tree you want to move.
2. Go to Tools > Change Organization
   
3. From the drop-down, select the Organization that you would like to move the tree to:
   

Please note: Once you move the tree, you'll want to use the Assign Collaborators tool to give access to the tree to any other authors/editors in the new organization.

If you only have access to trees in the SOURCE organization:

1. Open the tree you want to move.
2. Go to the Assign Collaborators tool, and invite someone with an account in the TARGET organization to be an Administrator on your tree.
3. Have them use the Settings tool (see above) to move the tree.