I plan to eventually move my coins to a Trezor wallet. But is there any way I can disable the old default wallet in my old PC, before I get rid of the old PC entirely?
Since you are going to buy a Trezor hardware wallet, just move all your funds into the hardware wallet with the new seed phrase generated as you set up your wallet and back it up. once you have swept your funds into your hardware wallet, you no longer have to disable the wallet on your pc or worry about it, because your funds are now in a new wallet, with a new seed phrase and has no connection with the old wallet.