As Neal correctly pointed out, if you're changing the line then just change it. It was mentioned that material is the same, so there should be no need to delete and re-add the line.
Not sure if the second question is a continuation of the same (and how it's related), but this error occurs because the mandatory pricing condition is, well, missing. If you go to the Conditions tab and click Analysis button, you might be able to find out why. Reach to a local SD expert for further assistance on this.
Overall it might be better to explain what is that you're trying to achieve in general.