We had this situation a couple of years ago. The railings on three balconies needed to be replaced since they were starting to show damage. Our Strata Manager informed us that since they were being replaced we were required to bring all the other ones in the building up to compliance with the latest standards. Our approach was to get new railings for the three affected balconies and then to install a bar across all balconies to bring it up to the newly required height. That met the standard, looked quite good, and was a lot cheaper than replacing the entire thing.
There are two buildings in our Strata. We did not need to do that on the other building since none of their balcony balustrades needed fixing.