I need 2323k iron to increase hunters lodge to L2. With maxed out iron storage and mines I could only store 2322K. I converted iron mine to advanced iron mine L1. I then upgraded advanced iron mine to L2. It showed I would gain an increase in iron storage (I just need 1k). I don’t recall the specific figure it showed but it was >1k.
Upon completion of advanced iron mine L2, max iron storage is still at 2322K