We cannot apply a Recurring Gift payment to a different Recurring Gift once the Recurring Gift payment has been applied to a Recurring Gift and saved. 

Alternative Solution:
1. Delete or adjust the Recurring Gift Pay-Cash to $0
2. Open the Recurring Gift record
3. Click the Transactions tab
4. Click the Rollback Transaction button (located in the lower right-hand corner)
5. Click Save and Close
6. Recreate the Recurring Gift Payment