Login. Open Keychain Access now—without the file restoration—and select the current Login keychain, and have a look around.
Do restore the old Login keychain file under a different filename, open it using Keychain Access, and have a look around.
I’m not aware of a supported keychain merge operation, though there may be third-party tools that can do that.
You could choose to clobber everything in your new Login keychain, if you wanted. I’d reboot the Mac after replacing the keychain, though. And I’d absolutely keep around a copy of the keychain that was created when you established this new login, on the off chance you need a password or a certificate or setting from that.
If this is a new install, using Migration Assistant makes all this vastly easier.