It’s not a huge change, and day-to-day the differences will be smaller things like words that are used differently. You get used to that without even realizing it. I remember feeling very pleased the first time I naturally used the word “jumper” the way Aussies do (meaning “sweater” or “sweatshirt”).
Aussies are generally friendly toward Americans, and thanks to Hollywood they tend to feel like they know a little bit about the USA which makes them interested. (When we visit my family in the US, my Aussie husband says he feels like he is in a movie. 😆)
I have never had any problem with people here not accepting me at face value despite being a foreigner and my accent giving me away. If anything, it’s a talking point when getting to know a person I haven’t met before. They’ll often ask because they are curious, but they aren’t hostile. If anything, they tend to be intrigued that I chose to live in their country instead of my country of birth.
There are systemic differences that may or may not be difficult to get your head around. For example, I didn’t find the health care system very strange, because I was young enough when I moved here that I hadn’t really gotten my head around how it worked in the US. But when my parents come here, they won’t consider going to a doctor if they need one because insurance, even when I tell them it’s a flat fee and give them the amount the local practice charges. It’s just not the system they know.
As noted by others, Australia has its own issues with racism. You won’t escape that by coming here, though it is different. Here it’s rooted in historical treatment of Aboriginals as sub-human, and “white Australia” policies from the early 20th century. Basically white people have a superiority complex wherever you go in the world of former European colonies.
I’m not sure whether any of that actually answers your question… Please feel free to ask more if need be.