If there has been no material change in any of the relevant conditions which the court must consider in determining which parent should have primary custody since its last order, it is likely that the court will allow that
order to remain unchanged. (The fact that the subject child is now 12 and may now wish to live with the noncustodial would not likely be sufficient reason for the court to change its current custody order involving
this particular child.)
Under Washington law, your son's mother would have to prove that a "substantial change of circumstances" in your life or your son's life had occurred that would necessitate a change in custody, AND she would then have to prove that a change in custody would not "detrimentally affect" your son.
This is heavy burden to overcome - and for good reason. The courts want to promote stability of its parenting plan orders so that parents and children alike can rely upon them. Only when drastic things happen will the court re-open a case to consider changes to your parenting plan.
I see in your question that you and your wife are actively involved with your son's life - you are good parents who provide a stable home for your son. You mention no underlying reasons for your son's mother to seek custody, other than to insinuate that your son can make that choice himself. Your son's age is not a factor in this - it is a misnomer that a child in Washington has the ability to choose who he resides with (even assuming that he'd choose his mother's home . . .). Your son may make this choice when he turns 18 but not before then.
Based upon what you have said in your question, I do not see that your son's mother has met the burden of showing that a substantial change of circumstances has occurred to warrant stirring up the parenting plan. I do not see that you are at risk of losing custody based upon the limited information provided in your question.
Now, does this mean you should wait to seek legal consultation unless or until she tries something? No - I would suggest you make a connection with an attorney you trust NOW so that if your son's mother does file some legal action then you don't need to quickly figure out how to deal with that situation while you are feeling the stress of the situation. A few ideas about how to do this: Ask friends and family for some names. Look through AVVO for Tacoma area attorneys. Make some phone calls from the phone book. When you speak with an attorney, let them know you are wanting to establish a relationship with an attorney you connect with so that as issues arise you will know who to call. You should expect to pay a consultation fee to an attorney you meet with, although some do provide free consultations.
You might find some helpful information on this self-help site too: http://www.washingtonlawhelp.org/WA/index.cfm
Another avenue for you might be to seek some form of mediation around the allocation of parenting time. Depending upon your situation, establishing a relationship with a mediator who can help you and your son's mother communicate more effectively over the next 5-6 years can reduce stress and anxiety for all concerned, which tends to reduce the desire to lash out by filing spurious legal actions.
Perhaps there is nothing to mediate at this time - perhaps you could engage her in a conversation now about what her needs are around the current schedule and see if engaging her in such a discussion reduces the threats she makes. I do find that often people in the situation you describe (albeit I have limited info about your son's mother) use threats and lashing out as an extremely inarticulate way of expressing a desire to have a better relationship with a child.
A mediator is certainly less expensive than an attorney, and can help educate both of you about the realities of the custodial situation (for instance, by helping her to understand that a 12 year old doesn't dictate custodial arrangements . . .). If this sounds like a route you want to try, take a look at this website: http://www.washingtonmediation.org/about.html
She will have a near impossible time unless she can prove that there has been a substantial change in your home that is so bad for the child that a change in custody needs to occur. The fact that the child may or may not want to change is irrelevant to her case. Your son gets to decide when he's 18. Before that, the Parenting Plan controls. As far as him not wanting to go, remember, he doesn't get to decide. The court looks at it like a kid not wanting to go to school. You have to make sure he goes or you get in trouble. You follow the Plan or you may be found in contempt. If you are found in contempt a couple of times regarding visitation for not making sure your son goes, that could be a be a basis for her to come to court and ask for a change of custody.
The key will be to respond in writing to anything she files with the court. File it and get a copy to her. The first thing she will have to show is called adequate cause. She will file her Petition to Modify and then must note a motion for adequate cause to show she has alleged enough to have a trial. If she loses that motion, the case is over. As a result, you will want to respond and file a declaration disputing her allegations. In her Petition, she will allege facts under a section of the form that also refers to a statute. Look up the statute and see if she has alleged enough facts.
I have had many cases dismissed because there wasn't adequate case, so hit it hard at the beginning. Also, many counties have a Family Law Court Facilitator who helps people without attorneys for free. Might want to get the contact information for this person in your county. Also, the state court web site has forms and instructions that may help.