Your place on the national senority list would be determined by your date of hire at the regional for which you are working when it is enacted. If you change airlines you are inserted into that airlines list based on that. So yes you could bump someone out of a seat and someone could do it to you if they were to come to your airline. Cuts both ways.
One thing that would have to be decided, and I don't know where I would stand on this yet, is this. Would this system apply if a pilot switches airlines on his/her own volition or is forced out? But I'm inclined to think I would want it to work in both cases. This way, just like in other businesses, if we want to change employers we can do so without having to lose everything we've worked to accomplish. This would also be benefical if a pilot needed to relocate to another part of the country for some reason. Instead of having to commute, if a regional in that area is hiring, he /she could just change airlines and be closer to home.
In the business world if you decide to change employers to do the same kind of work, you just switch companies and take your experience and knowledge with you. You could either move up or down a little at the new company but would be pretty close to what you left. Why should we be so different.
The way we do things is antiquated. We need some new thinking. Time to get out of this box we've built around ourselves.