I had this beautiful but unwearable sweater. It's nice and warm, but the wool turtleneck was a bit to hot and claustrophobic for me.
I marked a line down the middle of the collar to the point where I wanted my v-neck to end. I stitched a straight line down the mark, and a zig-zag on either side, and across the bottom.
Then I took a small sharp scissors and cut the collar open along the straight sewn line.
I hand sewed a piece of ribbon to either side of the cut edge.
At the "v" I mitred the ribbon so that it made a smooth edge.
Now I have a wearable warm sweater!