This will cause you to be the neutral faction, but perma overt to both Imperials and Rebels.
Go to your recruiter, and say you would like to resign. Wait for the confirmation window to pop up to type in "yes". DO NOT TYPE IN YES, YET!
After the window is up talk to the recruiter and get the place where you want to be special forces. DO NOT CLICK TO CONFIRM YET!
Go back the the resign window and type in yes, and leave the SF chat log open.
When you have resigned, click that you would like to be SF, and there ya go.
You're neutral and perma-sf to Imperials and Rebels.