We are all quite proud of Measure H, the 2004 county measure which banned GMO agriculture in Mendocino County.  Mendocino was the first county in the Western Hemisphere to ban GMO crops.  Measure H had a domino effect in which many other counties in the USA, and provinces and states around the world all followed suit and banned GMO agriculture.  In response, ALEC (the American Legislative Exchange Council) pressured states to prohibit counties from banning GMO agriculture. 

ALEC's response to cities and counties acting as laboratories of democracy has traditionally to crush it through "state preemption laws" that prohibit local governments from raising the minimum wage, regulating GMO's or building municipal broadband.  http://www.prwatch.org/news/2015/07/12876/alecs-2015-agenda

Considering that Measure H is but a measure from an general law county, it could be overturned if the State of California followed ALEC's lead and prohibited counties from prohibiting GMO crops. 

The most powerful advantage of becoming a charter county is: provisions in a county charter have the same force and effect as acts of the California legislature. 

The provisions of a charter are the law of the State and have the force and effect of legislative enactments. 
— CA Constitution Article  XI, Local Government, Section 3 (a) http://www.leginfo.ca.gov/.const/.article_11

If the new charter stated that Mendocino would be a GMO free agriculture county, it would have the same force and effect as state law and could not be overturned by the State. 

ALEC's response to cities and counties acting as laboratories of democracy has traditionally been to crush it, through state "preemption" laws that prohibit local governments from raising the minimum wage, or regulating GMOs, or building municipal broadband. - See more at: http://www.prwatch.org/news/2015/07/12876/alecs-2015-agenda#sthash.hyWKvMXF.dpuf