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?
Make sure that you permanently delete all files on your old computer, but if you want to be safer I suggest doing additional full format of your old hard drive, just make sure you have backup words in physical form.
Only way to move those coins from Electrum to Trezor wallet later is to make a transaction to new address generated in Trezor, but don't do it right now when transaction fees are very high.