Hey Kathy, 

Caroline here with Guru! 

To change ownership of a group of cards you'll want to change the verifiers.  It should not matter who created the card first.  To change the verifier you're going to want to follow the steps in this article. 

1. You'll want to filter using the "Filter by Attribute" column on the right-hand side of the screen to "Author = person's name you want to change." 
2. Once you've filtered down to just the cards this person created, you'll want to click on the 3rd icon at the top of the screen to change the verifier. 
3. Then you can select which person or group you'd like to take over ownership of the cards. 

Please let me know if changing the verifier is able to help you achieve what you're trying to do!