File > Document setup…
I suppose that Layout adjustment would not help you.
That is a dramatic change of size. I don't see how any automated system is going to help here.
Liquid layout required that rules be applied to everything on the every page and layout adjustment depends on how well the layout was done.
I'm not optimistic that this will get done without a lot of work. Not saying you have to go back and start from scratch but it could take a while. This will however be a good teaching moment about this kind of work.