Scenario 1
Before your lose access to Canvas, you can add a recovery email (e.g. personal gmail) which is not associated with any other edX account.
Proceed to reset password to set a local password for the same NUS email address.
Scenario 2
In case you are not able to access edX through LumiNUS, you may go to Sign In > Need help signing in? > Forget my password to set to a local password for the same NUS email address.
You will be able to continue using this edX account for new enrolments and access your previous courses and certificates.