After you lay a cable, when you go to change the cable you will put down the same cable as you put down before. For example:
I had HV cables running into MFSU's. I went to change them to Fibre cables, I knocked off the HV and placed the Fibre and the HV cable came back. When I broke the Fibre turned HV cable it gave me an HV Cable.
I also have another issue with sometimes when you break a cable, you'll see it gone but there will be an invisible block there and you have to go into MCEdit and delete it.