The register values shown in the 'Task stopped' window can be altered. The registers are set from this window when the game is continued. In the case of a SWI trap, the trap occurs after the SWI has been executed, so you can alter the registers returned from the SWI.
It is also possible to alter the PC value to make the game continue from a completely different place. You can even change all the registers and the PC to jump to another subroutine somewhere. But be careful with the status flags in the PSR; the program could get confused if you accidentally turn the interrupt disable flags on or put the PC into supervisor mode.
Putting the cheats into action