My ex is a Narcissist, too and loves to " surprise" me by changing the parenting schedule. We only communicate by email so he would send me long weird rants that I'd skim through and then find a little comment like "I'll bring the boys home on Monday" when we had planned for them to be home on Saturday. My favorite was, "the boys will spend every Friday and Saturday night with me (ex)" buried in a newsy email... he decided to adjust the parenting plan on his own and thought I'd just go along with it.
My ex also used to talk the kids into letting him into the house, or I'd find him "working" in the garage. Funny how many narcissists have the same behaviors. He still thinks this house is his, too! But I changed the locks, security system and now my kids are old enough to just say no to him. Yeah!
So we are making progress and I now keep my emails brief and don't even use complete sentences, just days and times and my ex does the same. Even so, I know there will be more surprises in the future. For me, the key has been to not engage my narcissistic ex. When he has a schedule change that doesn't work for me, I just reply, "nope". You know, once you include any kind of explanation, you are in for a long interaction. Good luck!