you can't run a 20x20. The 15cm depth is the physical max. If you physicaly move the carriage to the front of the case - the extruder nozzles are bang on the edge of the plate. There's no wiggle room there.

But you could run a 30x15 plate with no problem. Like i said you just need to print right with right and left with left.
Short of actually making the case bigger and moving the guide rails - there's no other way to do it.
Well you could refit it with a single extruder assembly I suppose.
Which would give you the 30x15 for the one head. Make life slightly easier.
But as the design stands you'd need a total rebuild you get any more y or z capacity.

End stops don't need to be moved.

But the hard part is always going to be getting the software to use the larger build plate.