Im looking at replacing mine in the future as well and Im just as confused as you are regards what fits what - and to be honest Im probably going to unbolt mine when the time comes - and take it into bunnings to be sure it will work hahaha (yeah might be a little embarrassing hahahaha, but my experience with Bunnings advice has been great sofar and with any luck you end up talking with a retired ex tradie plumber in your case, and if any adapters are required for the new cistern it might be handy having the old cistern there)
Apologies for the slow reply, but I wanted to reach out and extend a very warm welcome to the Workshop community and see how you were going with your toilet replacement. Do you still need a hand? You might be interested in this toilet installation offer from Bunnings, which uses a fully registered and licensed tradeperson to install your new toilet.
Please feel free to post on Workshop whenever you need a hand with a project around the home or garden. We have loads of community members regularly sharing their knowledge and inspiring projects. And please let me know if you ever need help getting the most from the site, or have any feedback about how we can improve Workshop for you.