I dont think the Bubble Dome in Melbourne can now be upgraded to meet the minimum 40,000.
There's a contract in place between the Victorian Government and Etihad Stadium that prevents a 'similar size' stadium from being built until 2025 or so.
Hopefully if Australia gets the bid this will be fixed, BUT Etihad Stadium CEO Ian Collins (who was once President of Carlton AFL team) did say that he's "open to all offers" regarding use of Etihad.
I believe that the AFL will take FULL ownership of Etihad around 2025, 3 years after 2022.
Etihad was meant to be rectangular for the Sydney Olympics, but the AFL got involved and sold their previous outpost of Waverley Park to part-fund Etihad.
The decisions on Melbourne's stadium made in the last 10 years were before soccer's re-growth, hence the dilemmas you see today.