I recently moved from Australia to the US and have purchased a US WiiU. To access my digital-Australian games I'd need to change the region on my account from Australia to USA, but I'm worried that I might lose stuff. Especially considering I would have to cancel my Nintendo Online subscription to do so. Will I actually lose anything? Games, save data ect.?
No. You won't lose anything because the Switch uses a “Nintendo account”, and the WiiU uses a “Nintendo ID account”. Even if they are linked and you change the region on the Nintendo account, the Nintendo ID account doesn't seem to change. Also you don’t have to cancel your Nintendo Online subscription; you only need to disable “automatic renewal”. So as long as you're still covered under the subscription you shouldn't lose any game data. As-for games, if a particular game isn't available in your region it will be "hidden" but not lost, it's still attached to your account, you just can't play it without switching your account to a different region that does support that game.
On a side note, the WiiU store is region locked meaning if you have a US WiiU it won't let you change to the PAL, or Japanese E-shop.