Perhaps set up new document in A5 first then cut and paste from A4 doc. Check margins though but they should be set so the wrap should be fine. Lot of work for 5000 pages but "select all" might help. I then would check word wrapping by selecting all again in new doc and then moving margin tabs around but back to orginal setting for A5 and Word should align everything ok. Done it many times but somewhat smaller docs.

Couldn't reproduce your issue but then I have nothing like 5000 pages to format.