The change in the constitution only makes it harder for a...

The change in the constitution only makes it harder for a company to do a partial takeover without asking for approval from the shareholders.
A partial takeover can still happen, but now it must be put to a vote, and therefore to succeed it would likely need to be a better offer. But even if it isn't a better offer, minor shareholders who otherwise wouldn't have known about the partial takeover have an opportunity to get out, that they wouldn't have otherwise had.
Some interesting reading is here
https://www.allens.com.au/pubs/itd/aug02/tre.htm

So there is nothing stopping CAT or someone else wanting to take a significant stake. But now we will find out about it beforehand.