When you load the new filament, it will just push out any remaining filament during your normal purge. All you need to do is load the filament into the extruder's drive gears... that's the reason we back out filament before cutting it... that gives you the space that you use to load the new filament... then in your purging, you purge enough to allow the color to transition in the purge tower.
All of our profiles already do that when they involve the cutter.
Follow these steps: