Generally, you are not required to bring a building (or playground or whatever) up to current standards but 1) you are required to maintain it to continue to comply with the standards that applied at the time of construction and 2) if you change anything substantial, the new bit has to meet current standards.
Generally, your insurer will have these requirements as well as any authority giving a development approval. That said, if something is clearly below current standards, it might be reasonable to make improvements to meet the current standard. Some deficiencies relative to current standards are pretty minor and the risk to safety can be judged to be small. Other deficiencies really ought to be brought up to standard, especially if the risk is substantial and the cost to remedy is not large.
I would think it would be hard to argue that someone should be prevented from bringing their railing up to a recognised current safety standard but it is similarly not reasonable to require it of everyone.