I saw the "Apply properties to all conductors of this potential" flag but if I uncheck it, change the properties and enter it again it is enabled again (tried with 0.100.0-dev and 0.90).
It's deliberate!
Its for safety reasons, For security reasons, you should deactivate this checkbox each time you modify conductors. For example, adding a new wire with a different section in the equipotential wires..
Edit:
ps : I hope we've answered your question, which isn't easy to explain why we've put this in the code. I also hope we haven't offended you or questioned your skills as an electrician, that wasn't the point, if it was I apologise in advance.
Best regards,
Laurent