My Developer ID certificate will expire in few days, so I downloaded and installed new certificate in the login keychain. However my key is still linked to my old certificate. I have my .p12 file but even if I delete the old certificate from login keychain and reinstall the .12 file, my old certificate reappears in the login keychain.

I tried to select the new certificate in the login keychain and choose Files > Export Items (in Keychain Access) but in the Save dialog under File Format the "Personal Information Exchange (.p12)" option is grayed out. How can I generate a key/.p12 file that will be linked to my new certificate?