If you've converted the curve to an angle or simply just have angle points, press Shitf+C or select the Convert Anchor Point Tool within the Pen Tool buttons, then click on the anchor point and drag it to turn it back into a curve.
Conversley, if you have curves and you want them to be corners, you can again use the tool to click on the anchor points.
I hope this is somewhat close to answering your question.
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