Loco change was definitely at Ricky, I don't think that Moor Park has ever had sidings that could handle the change, Ricky still has them at both ends of the station.
The publicity for rover tickets says that they could be bought up to one week in advance but as they were hand stamped in the 60s there would be nothing to stop them being dated further ahead if the clerk felt like it.
I think it was a reference to 'Twin Rovers' not being available northwards (on the Aylesbury line) beyond Moor Park, rather than to the loco change?