A member of Decode Ingress community Shane Pitre found this great trick how to convert Old to New Shields:
This is 3 step process:
- Drop all your shields
- Force sync
- Pick them up
Even if it is 3 step proces it can last few hours depending on how many step 1, and step 3 you need to do. Also you can do this for separate type of shields, do first common ones, than rare and than very rare, it can be easier to you.
Good luck and enjoy your NEW Shields!