The RAAF looked at re-engining the Caribou with a turboprop many, many moons ago but I believe they ran into all sorts of CofG problems with the lighter engine.
On top of that the problem isn't just the engines - the RAAF has had problems sourcing other components for many years. I dare say it's all becoming a tad expensive keeping the old girls flying. I guess it all comes down to whether or not the Caribou's full capability is actually required.