Not sure GRBL support tool change automaticly.
The path of least resistance is what Chris suggest, its also what I do myself.
Run first program, return to G28 if selected, change tool, reset Z and load 2nd program.
You could insert a G28 and dwell command (or press pause?) manually in the gcode, but it isnt very slick
Not sure you could reset Z during this pause though.
Mach3 and ATC may be in your future