We can't answer your question because we don't know enough of the details. The question is not as simple as you think it is. Depending on the nature of your business, how you were paid, and what understandings you had between you, you and your partner might have been self-employed before you formed the LLC, or you might have already had a partnership, at least for tax purposes, even if you didn't have a formal partnership agreement. And you can't even begin discussing how you make the transition to an LLC until you clarify what you are transitioning from.
Washington does not have a personal income tax, but it does have a tax on gross receipts of a business, the Business and Occupation tax. Your partnership or LLC is a business, so you are going to have to pay the B&O tax.
So I will second Critter-3's recommendation that you meet with a local tax professional who can review your entire situation and figure out how you have to handle it. You are not going to get the answer here.
