I think the problem is not that they are switching to a less expensive Champagne, it is more that they are doing it ON THE SLY.
They still advertise the champagne in the menu as being the Dom.... and there has been numerous documented experiences in this thread of crew trying to hide the fact that they did not have Dom on board, or making up stories that it is suddenly only for welcome drinks etc. It is obviously embarrassing for those crew members that the company is doing it this way.