Your last sentence "If you want to use the same mesh as the foundation for a new mesh, you can duplicate the mesh, rename it so it uses a different image, and then edit the vertices as needed" is exactly what I am trying to do.
I have two characters which are extremely similar. They have slightly different contours but the exact same assets in general. It's just two cats. In the past we have always been able to take a scene that has two extremely similar characters, re-link the image path to a new image folder with different images (again, very similar images but with slightly different contours), and then make small modifications to those meshes to adjust for the difference in contours. Of course this means that we do not have to completely re-do meshes, weighting, and animation for two nearly identical characters that need nearly identical animations. Which is great.
This time, initially the meshes were totally locked, and now as you see they are still not working correctly. This is a brand new problem we have never faced before. This exact same process has always worked for us in the past until this new update...we've done it literally hundreds of times.
UPDATE: Ok so I did figure out a way around the issue, but it does in fact verify that it is an issue with the update. I was working in the newest version- that is 3.8.96. I realized I could try to revert to a previous version, and then try to use "import project" to bring the newer scene in rather than actually trying to open it. I did that and am pleased to say I was able to get the scene into the reverted version. I re-did the process of relinking the images and this time it worked great. No locked meshes, no sticky pixels. So yeah, it is definitely something with the update. For now we will all revert and stick to the old version...could you please update me by email when you know the issue is resolved so that we don't get too far behind? It would be very much appreciated.
Sorry that I didn't bring up the relinking of images in my original post- if that was the cause of the issue then I can see where it would have steered you wrong.
Thank you for your help 🙂