Pls, find code..

trigger AccountAddressTrigger on Account (before insert, before update) {
    
    // Process only for before insert or update
    if((Trigger.isInsert || Trigger.isUpdate) && Trigger.isBefore) {        
        List<Account> newValueList = Trigger.New;
        Map<Id, SObject> mapOldItems = Trigger.OldMap;
        
        for(Account acc:newValueList){
        
            Account oldAccountObj = null;
            if(acc.Id!=null && mapOldItems!=null) {
                oldAccountObj = (Account)mapOldItems.get(acc.Id);
                system.debug('Old Country: ' + oldAccountObj.BillingCountry);
                system.debug('New Country : ' + acc.BillingCountry);
                system.debug('Old Country Code : ' + oldAccountObj.BillingCountryCode);
                system.debug('New Country Code : ' + acc.BillingCountryCode);
            }
            
            // Check if checkbox is true
            if(acc.Match_Billing_Address__c) {
                acc.ShippingPostalCode = acc.BillingPostalCode;
            }
        }        
    }
}

Good part is now I see I am getting updated Country Code...for changing country name.

|USER_DEBUG|[13]|DEBUG|Old Country: United States
|STATEMENT_EXECUTE|[14]
|HEAP_ALLOCATE|[14]|Bytes:14
|HEAP_ALLOCATE|[14]|Bytes:27
|USER_DEBUG|[14]|DEBUG|New Country : United States
|STATEMENT_EXECUTE|[15]
|HEAP_ALLOCATE|[15]|Bytes:19
|HEAP_ALLOCATE|[15]|Bytes:21
|USER_DEBUG|[15]|DEBUG|Old Country Code : US
|STATEMENT_EXECUTE|[16]
|HEAP_ALLOCATE|[16]|Bytes:19
|HEAP_ALLOCATE|[16]|Bytes:21
|USER_DEBUG|[16]|DEBUG|New Country Code : IN