Convert A3 to A4 format

[Rod Fee]

Hi team,

I have a novel that I set up with several master pages for different files.

The majority of these have the following frames:

-Footer
-Three text boxes that flow to following pages
-A header column for each text column

I originally set it up as A3 because I thought there would otherwise not be
enough room for  meaningful amount of text in each frame on each page. 

I was wrong. Now I want to make it all A4.

When I select the files and try to change the page size to A4 it tells me
the column sizes will be all wrong  - not entirely sure how to fix this
easily. Any clues?

Regards,
Rod Fee