If one cuts a second bulkhead the coupler can be turned into an ebay and the motor mount modified for H motors. Stock, the rocket can handle the H128 but don't go any higher unless
the fins are reinforced. They'll flutter otherwise. Kurt
Just open the .rkt file in Open Rocket. Then file -> save as a .ork file. IIRC, there is a drop down box to switch from .rkt to .ork in the save dialog box.