Maybe I've missed it in discussion so far, but there are injection and pour-in foams that are very different from spray foams. They tend to be fragile, brittle, less resilient than spray foams. There are several different products within this class, with more coming to market. Some have a significant amount of water, which is supposed to vaporize due to the heat of reaction when the foam is installed, but if not vented somehow, it will re-condense somewhere within the structure.
As for installing them where FG is already in place, they are marketed with that effect in mind. The installer is able to insert the tube and the foam pushes the fiberglass out of the way as he pushes the tube in. It fills the wall cavity as he withdraws it, packing the glass down fairly tight. This is an advantage over blown dense-pack cellulose, which won't compress FG and will hang up on obstructions.