A Frontierland CM told me while we were waiting for my kids in Splash Mountian.
She said 8-10 weeks "around Sept" to change the vehicle for safety reasons. Sorry I don't have dates, however.
So far I have looked on official Disney site, this site, www.wdwig.com, and other sites.. and as I read it.. it looks like September.
I have not heard an "official" word on this either. Yes, CM's may know, but sometimes they are the last to know.. so I take what I hear with a grain of salt.
A remember most of the ride ops are "worker bees", which then reports to a lead ride ops, then a ride op supervisor, then whoever is over that, and whoever is over that, and then you may get to the assistant manager of the park. What I am trying to say is there is multiple levels of management from the ride op on up... and sometimes they don't let any of the info to the ride ops....