I would argue that converting elites is a gain associated with trad march, and therefor should be part of the "No Gains Penalty" hitting into Prosperous stance. As it stands the stance was designed to have 0 protection for going into it, and instead its hey, just kidding you actually get protection from about 62% of the attackers who need to made trad hits for conversion...
Granted it is probably too late to do anything about it now, but details should have been spelled out. You say that it was just a name change and gains change, but the Forum Post says Fortified Stance Removed, A new stance called Prosperous will replace it, not Fortified changed to Prosperous. (In addition to just the name and gains mod, there was also a change to sci production in prosperous).