What we are going to do is remove three courses of existing shingles at low slope interface and add TPO membrane flashing back under the roof felt and over the top of the existing low slope flashing. Always use a membrane cover tape to make the TPO stick to the metal surface of the sunroom roof deck. Then add new shingles back over the membrane.
This should stop all leaking at the interface of the shingles to the low slope roof of the sunroom. This solution should save our customer a lot of money by not having to put membrane over the entire sunroom roof.