Duty cycle is in conection with common planes/multiplexing ratio. If You use "static" LCD (numerical or custom one with extended temperature range) You have one common plane and duty 1:1 . To keep pincount low - multiplexing is used - more common lines , multiplexing, lower pin count, complicated liquid crystals. In Your case changing setup will result in false work of LCD - routes , crystal and controller are connected by LCD designer with these limits in mind.
regards