How do i change bits and pick up where I left off
You will need to do a two stage carve, Look for Paw Paws Workshop he has a video on it on Youtube. Be sure to set you machine up to lock your motors this will keep you from accidently moving the gantry and losing you starting point.
And also set switch #4 on each of the three red switchboxes inside the Xcontroller to OFF. This deactivate current reduction mode for inactive steppers so they will have full power at all times.
Also changing GRBL parameter $1=255 dictate that steppers never go to idle (in terms of power)