It can be easy to be sucked back into the N-vortex when they shift their tactics. I have pondered just a bit into why we adult children of narcissists (ACONs) get suckered by our N parent(s) when they suddenly seem to be "nice". I think at least part of the reason is that we are honest-hearted people. I have noticed that honest people don't presume deception in others. Especially in our parents or close relations. More on that in a moment.
Here is the scenario I'm picturing as I write this: you've been used and abused by the narcissist for years. You've started to get a clue as to what you are dealing with. You start establishing some boundaries, you start using a backbone, you are putting up with less crap. In the wake of the changes in you, the narcissist will also appear to change. They take on a more submissive posture. They seem to be behaving better. You start to have hope that you've made a difference. You start to believe that the narcissist's character has changed. You are set up for a fall. It is typical for a narcissist to back down when their source of narcissistic supply shows strength. They are bullies. Bullies back down when challenged; bullies start to whine and cry when someone stands up to them. That is what the narcissist in your life has done. They have not changed who they are, they've only changed tactics. What they can't get by baring their teeth they will attempt to get by showing a "softer side". They may suddenly appear reasonable. The rages stop. Or they will make a great show at being helpless and pitiful. They may suddenly get religion and make a profession of godliness and make a great show at waxing their halo. Whatever the new, softer version of narcissism you see in front of you, know this: it is still a narcissist you are dealing with. The fangs are still there....they've just hidden them for effect.
When the N makes changes in behavior they simply are employing a ruse. Ruse = deception. They lie. They lie by omission, commission, by a look, by a sigh, by insinuation. They are the personification of a lie. We, the honest-in-heart, have a hard time conceptualizing someone who exists entirely in a lie. It is not socially acceptable to allow our first presumption to be that someone is lying. Especially when it is a parent. No, we are to presume they are representing the truth...and only accept that it was a lie when it can absolutely be proven to be one. Once a lie has been proven you then have to shift back to the default position of presumption of the truth. It is this default position that screws us up over and over.
The sad, yet absolute, fact is that the default position when dealing with a narcissist is that they are lying. The anomaly with a N is when they tell the truth. They so seldom tell the truth that you don't even have to worry about mistaking that truth for a lie. They don't deserve to have us believe one thing they say or insinuate. If we presume that whatever they are doing, whatever they are saying, is designed to manipulate and deceive, then we are in a much safer and saner position. So, whether they are being "nicer" or whether they are raging, they are the very same animal. Predatory. The predation NEVER stops. Presume the fact that every waking moment they are predating you, and you'll always be on your guard even when it looks like they are now "safe" to be around. Never stop watching your back.