You shouldn't lose access to apps that you have downloaded to your devices in Australia. However, you likely won't be alerted to updates for any of those apps. Should you hear about an update, you would only be able to update them if they are also available in the US store.
You will have to cancel all Aussie subscriptions before you can change the country to the US.
Change your Apple ID country or region - Apple Support
M1 Mac mini, macOS 14.6.1; Watch, series 7 GPS/cellular, watchOS 10.6; iPhone 12 Pro, iOS 17.6.1; TV 4th Gen, tvOS 17.6; HomePods, audioOS 17.6 and Airport Express w/ Airplay 2