I’ve been testing one of my recent characters and have linked that character into a scene file. While working on it I realized I needed a new bone and bone constraint. I went back to my original file, added the new bone, made the new bone constraint and modified the rigify script so I had a custom slider.
When re-opened the scene file, the new bone and modified scripted updated just fine, but the bone constraint does not show up.
Just save your action for sure. Nothing will happen other than you need to proxy the rig. Then select the actions. All will be the same unless you have constrained one of the bones with a key frame. Simple matter to just select it and check.