Again, we are not a developer board, but a fan board...
Below is what I thought...
Original RBO: stage 1 - 9, largest filesize
ExScenario1: Arena mode, Ex-Stage 1 & 2
ExScenario2: Ex-Stage 3, 4 & 5
ExScenario3: Ex-Stage 6, smallest filesize
When we play stage 1 - 9 in ExScenario3, it will use the files in the original RBO...
Thus the expansion was designed that it won't run if all the previous version wasn't installed...