They normally update the indices every three months but perhaps...

They normally update the indices every three months but perhaps they make an exception for large IPOs in the ASX20 and do an out-of-cycle update.

But the index funds know MPL is coming in the next update and they know roughly how much they will need so I very much doubt they're waiting for it to become official. If they see value in it now they'll be buying now.

So there may be some minor buy pressure when the update happens, especially because at 2.40 MPL represents a somewhat larger slice of the ASX20 than at 2.00, so they may have to pick up more than they bargained for. But I agree with emhurley that most of the funds are buying big already, in anticipation.