Why on earth would anyone wind thread over existing thread on a bobbin? Use a new bobbin. Pull old thread off a bobbin, use up old bobbins for quilt piecing!

I did as a teenager because my mom did (we only had 2 or 3 bobbins total). I don't remember a problem with the "over-winding".

But now I don't do it. Dealer said to never do it (they also repair machines, and have seen damage especially to the sensitive newer machines). I have over 30 bobbins, so I'm not hurting for "free space" there. I use any color available for piecing, and try to keep a few medium greys and a couple of white wound for the ready.