The LCIA interface is not practical for creating a new impact assessment method that may contain hundreds of flows. It appears it is not currently possible to cut and paste from one impact assessment method into another.
I tried exporting to Ecospold and using a text editor to edit many flows at once, but I was not able to reimport the LCIA method I made from the edited Ecospold file.
I saw the post from 2019 about exporting and editing a JSON-LD file, but that seems really cumbersome. I suppose it is what I'll have to do if there isn't another solution, but hopefully someone can offer an easier way. It's really surprising this should still be so hard.