When designing with BL653, is it possible to design a single PCB which can swap in either module variant: 453-00036 (internal antenna) or the 453-00041 (trace pin) variant or are different board designs required?

Yes, a PCB board for the BL653 can be designed so that either the 453-00039 (integrated antenna) or 453-00041 (trace pin) can be populated to the board. This is how the DVK-BL653 board is designed. The internal antenna variant of the module is disconnected from the trace pin via discrete populations on the module. The schematic for our DVK-BL653 can be accessed from the BL653 Product Page in the Technical Drawings section under Documentation for additional reference.