I have traced the rule and it's still returning "StandardTimeZone must stay".
But, identified that the pyStandardTimeZone value for existing Calendar instances is showing null in the database table of Calendar, even though the value is selected from the dropdown with Europe/London timezone for all existing instances(pyStandardTimeZone is blank in tracer pages)..
Also, while doing Save as or Creating new Calendar rule the pzInsKey is different for both existing and new one.
Thank you very much for the information, now the timezone column populating the values in DB table for older and new calendar instances with the above steps. We are in a way to discuss with our team for further steps.