I am trying to change an existing economy advantage award booking PER-SIN and SIN-BKK with a two week stopover to one business advantage award ticket for 65k miles which is the same price as PER-SIN business advantage and see availability online for both legs separately and together but when i try to change it via chat the agent apparently only see "The Perth to Singapore flight is only available for rebooking while Singapore to Bangkok is available only for new booking." which strikes me as strange. I chatted with two agents on different days and both said this even as on both days it show edavailable online for new bookings. As I was able to book the same thing with stopover as one economy advantage ticket for 37k miles so the routing should be fine unless there is some route restriction that only applies to business class. Earlier I had tried booking SGN-SIN and SIN-PER with a two week stopover as one business advantage award ticket which showed available and could only waitlist despite showing immediate availability which I assumed was some transient glitch but I'm not sure if there is something else going on.

Anyone run into similar issues or was able to successfully book a similar stopover award?