By saying you changed the lock screen to "swipe up," do you mean you currently do not have to enter a PIN or password to get into your phone? What about after a reboot?
If the answer to both of those is no, that means your phone currently isn't secured. Asking for an existing PIN is needed to change or remove said existing security measure, to prove you are the device owner. I doubt it would ask for that" a non-secured phone. I'd make sure that isn't asking you to set a new PIN as part of the new security lock. In that case, there should be a way to switch that you ask for the new password.