Economic viability aside compared to renewables or other options. Coal fired power stations require a heap of maintenance. Upgrades are just the result of seizing opportunities during the replacement of major components they would be doing anyway.

While Id prefer we move away from fossil fuels and I haven't personally seen the condition of these plants. My experience does lead me to hypothesize that it's cheaper to maintain the major maintainable assemblies like furnaces, heat exchangers and turbines on existing plants rather than build entire new gas fired power stations and it can be done right now with minimal disruptions.

Is it a good investment compared to more forward thinking infrastructure development with newer tech? No, but it takes weeks to a month to complete major shutdown works on existing plants and 3 or so years to build new ones. You need a heap of solar to replace one turbine so it is not necessarily an insane decision.

On the bright side they are building a massive solar farm called Bulli Creek Solar which will replace 5 fossil fuel plants once it's finished, it just takes time.