It was voted on by the members and changed. This is not the first time the AUF has been a limited company. They were decades ago, but decided to change back. I would have to trawl though my magazines to find the saga, but I don't think they bothered involving membership back then. As an Incorporated Association (what it has been for most of it's history) it has to be Incorporated in a particular State and subject to that State's rules. I vaguely remember them being incorporated in Vic or NSW, but as a general rule of thumb, National organisations were Incorporated in the ACT, where they have been for years.
There was a lot of discussion on the pros & cons on this forum when it was happening. There were rules about where the Secretary could live (had to be in the ACT) and reporting - audits etc. So some positions were designated to be held by employees rather than Board members to comply. The reporting was a bit of a joke. The Company structure is National, so you don't have to be based in any particular State.
For members, little has changed. We still have Board members - although fewer and with more legislated responsibility - we are still members, but limited by guarantee (I think that's $1 each), we are still flying, the failed audits and forced groundings are behind us, for those who "just want to fly" the weather is more of an issue than RAA being a not-for-profit company.