Expungement doesn't really mean much anymore regardless of where you are, it seems. The records never go away and it seems like for a number of intents and purposes you're still seen as a criminal, even if you had the record expunged, or had the charges dismissed after the fact. Electronic records and the Internet make this situation even worse. There seems to be no true clearing of your name anymore in this country.
If it's a misdemeanour, you can file a PC1203.4 motion to dismiss and expunge, and I believe through that you can also get a reduction to an infraction prior to dismissal; this assumes you plead nolo contendere; I'm not sure you're eligible if your plea was entered as guilty or if you were convicted at trial, bench or jury. But you need to consult a lawyer, certainly, to find out what exactly is what and whether that would even help you in Washington (and you might need to consult a Washington lawyer for that, or at least someone familiar with WA law).
I'd recommend my lawyer, but he doesn't usually do post-conviction work, which is interesting since his firm does a lot of gun rights and defence against CA gun laws type work.