How to Identify items in a list with a -1 then change that item

I want to be able to get the items with a Z(0,0,-1) value and multiply the whole item by -1 to get the opposite value. Then append that new value into the same list, changing only those with a Z(0,0,-1) value.

If you’re dealing with “planes”, then check this demo. (12.6 KB)

Your solution gives me the below:

While i’m trying to achieve:

where y should follow the line

I actually might of solved it through another question you answered. Will confirm tomorrow.