Word document corruption after Windows 10 installation

After installing Windows 10, I reopened some Word documents that have been an ongoing project for the past 8 years. They are long and complex documents including images, artwork and Chinese characters. I found that the formatting of these documents has been completely ruined! Everything has been moved around, some characters have been layered onto of one another and to top it off, the two fonts used for the entirety of the documents have been changed to another plainer font. After checking the Word fonts, lots of them have disappeared, including the ones I use, and when I tried to import one of the old fonts in from a back up file, Word simply did not recognise it!
Does anyone know what has happened? Does Windows 10 automatically update to a newer version of Word when you install it? Is there any way to revert back to the way these documents were before the update? Manually reformatting them will take an estimated 1-2 years of work
I tried looking at doing a system restore but there are no pre-saved restore points so haven't had any luck there, don't even know if it would work anyway...
Any help or advice would be SO appreciated

First of all: The upgrade process does not change your documents. They only are not displayed correctly currently. If you have not overwritten the documents after doing the upgrade, you should really make a backup of these files! They then most likely will still be perfectly OK - but after you save changes on your Windows 10 machine, Word might well save the changed fonts and the broken layout, which then really ruins the file.

According to what you write I believe that you have to make sure that the missing fonts are properly installed in Windows. Installing the now missing fonts might already solve the biggest part of your problem.

What you need to know is the exact name of the fonts, which your document is using. If you do not know them, you can open a copy of the docx file with a ZIP program. Then look into the file word/fontTable.xml; search for "w:name" to get the names of the fonts, which are used inside the document.

If you came to Windows 10 by doing a Windows upgrade, then you will still have a folder C:\Windows.old on your system. The folder C:\Windows.old\Windows\Fonts should still contain a huge number of fonts, which you had in your old system. It is very likely that the now missing fonts are still in this folder.

There are two ways in which you can install a font: The first is a simple one. Just copy the font to the Font Directory located at C:\Windows\Fonts which will install it for the system to use. The other method is to click on the font file at its current location. It will then give you a screen that shows the font preview and there you will see the button to install the font right at the top of the screen toward the left side.

Thank you so much for your response! I'm not able to follow your instructions right now, but I will be back on the case tomorrow and will leave some feedback asap :)

Okay, so I've had some luck with finding and installing the fonts that were missing. The formatting instantly changed back to not exactly, but something much, much closer to the way the files were! 1 -2 years of correction work has now been reduced to only about 1 month of fiddling, THANK YOU SO MUCH!! I really, REALLY appreciate your help

That is great to hear.

Changes might also come from other sources, e.g. the normal.dotx file might have been changed. (I consider this not to be the case for a Windows upgrade, but just to mention: A change in normal.dotx can also cause documents to display differently.)

Something else in fact is the printer driver you are using (and the Windows upgrade may well have changed that driver).

When Word displays a document in “Print Layout” form, it actually uses the characteristics of the currently selected printer to determine what the layout should look like. Obvious items such as default margins, paper size, and other charaterstics of printers in general can have a document appear quite differently when viewed, or printed, on one system versus another.

I don't know, if this is your problem, but it might be worth investigating, if another (a newer) printer driver or adjusting settings in the printer driver maybe fixes things for you.