gistfile1.txt

    public List<Database.upsertResult> upsertWorkaround(String sobjectType, List<SObject> recordsToUpsert, Schema.SObjectField externalIdField, Boolean allOrNothing) {
        if (sobjectType == 'Account') {
            return Database.upsert((List<Account>) recordsToUpsert, externalIdField, allOrNothing);
        }
        
        if (sobjectType == 'Contact') {
            return Database.upsert((List<Contact>) recordsToUpsert, externalIdField, allOrNothing);
        }
        
        // etc
    }