As far as I know, you have to pay to get upgraded, and the cost for a last minute upgrade is very reasonable. When flight is full; I have seen non plus people are seated in plus seats (I don't know if WS has developed any structure to determine who get bumped up). It does appear that the FAs know who the plus fare people are as they dont offer plus food etc. to the non plus fare people - how I know, one of my neighbour asked for pringles after I asked for it and the FA simply smiled and say "sorry, menu items are free for our plus fare customers; I am happy to serve it to you but it will be $???." I have been on flights where there were only two people in the plus area and the rest of the plane were filled, may be one seat empty; FAs were not allowing people to relocate to the plus rows without paying.