Ideally this would have to happen earlier than the 1987 election, but there's maybe a window just after.. perhaps demoting those figures in the first post-election cabinet reshuffle?
The dilemma for Labour was that they could stave off some divisions for a bit by going full-bore Rogernomics but that would be very unpopular. Labour also would have trouble disowning the reforms, and still had the recession to deal with. Maybe Palmee keeps a lead in the polls by disowning Rogernomics, or Moore takes power right after Lange.
Right after Lange resigned and Geoffrey Palmer became PM, Labour had a huge boost in the polls and led national for a few weeks, before going down to even worse poll numbers. Is there any way Palmer's honeymoon poll bump could have been kept?