Really great question! It's definitley possible to change the shipping weight conditions. If you update the store's General settings, to set the weight (from Imperial to Metric, or vice versa), you have to recreate the shipping weight/rates again from scratch. So for example, if you set up the shipping weights when your store was still in lbs and have now changed your store to KG like you mentioned, you will need to make the shipping rates again.
Existing shippings rates will always display in the weight unit that was set at the time the rates were created.
I hope that helps explain why it won't change now but that it is possible to do so.