Aguila Blanca and sigcurious:
OK, so I found out more about transitioning from concealed to open carry in Washington. I was wrong about the law as I initially stated it; the law simply is as stated above.
The law quoted by sigcurious is the WA statute that limits open carry (RCW 9.41.270). Arrests and subsequent court rulings have both clarified and obfuscated the issue. As a result, many experts recommend keeping the weapon completely concealed when CCing and also exercising a large amount of discretion when OCing. In the words of one local expert:
In an urban setting, you could be cited under RCW 9.41.270, depending on your behavior; that is, if your behavior "warrants alarm". Enforcement of the law is weighted toward a subjective judgement on the part of the observer, whether a private citizen or police officer, and NOT in the intent or action of the armed citizen.
When I was advised about transitioning from concealed to open carry, the advice was based on avoiding undue attention and possible arrest, and not based on a specific prohibition in the law.