IOS – Taking over old iPhone 4, updated to iOS 6, but can’t activate

You can't. Not only is there no way to unlock the new baseband, but there's also no way to downgrade it. You are out of luck.

This is most likely the carrier intentionally deactivating the old SIM.

When iPhones first came out, your current phone would normally get deactivated at the moment you purchased a new phone and told the carriers to move the service to the new phone. Now, they wait until the new phone checks in to perform the swap to account for delivery delays, backups, etc...

Your first step should be to contact the carrier and explain that you want service on both phones and arrange for which number rings to which device. They might be able to accommodate this over the phone, but if the old SIM has been deactivated in the networks, most carriers are very unwilling (or unable) to re-activate the old SIM and instead ask you to get a new SIM from them at a convenient local retail spot or by them shipping you new SIM card(s).

Apple's restore process and activation of the device doesn't affect network activation and for locked phones, you just need a SIM for the correct carrier to activate the device. iOS does not check or care that the contract is expired, not established or live for iOS activation - just that the SIM is manufactured for the correct carrier.