Can severe dysplastic Nevus turn into Melanoma after a year not being treated properly?

I have had melinoma insitu 4 years ago and also have regular skin checks every 6 month with recurrent severe and moderate dysplastic Nevis. At a recent check up I was told that I had a severe dysplastic Nevis on a shave biopsy that was done over a year ago.. I needed to have the borders removed and this wasn't done. It has been done now but I'm completed scared and wonder how could this have happened. Do this spots change rapidly and would there be a sign that it was growing?