Upgrading to High Sierra caused the Times font in MS Word documents to change, to become thicker and darker and less well defined. Can this be corrected?

Possible Fixes for High Sierra

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Resetting your Mac’s PRAM and NVRAM
3. Reset the System Management Controller (SMC)
4. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
5. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
6. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button below Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
7. Download 10.13.1 High Sierra Update and reinstall it.
8. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
9. Erase and Install OS X Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Install OS X and click on the Continue button.

Office 2008/2011 for Mac installed their fonts into System font locations. This included the MS version of Times, and others. When you installed High Sierra, it wrote its version of Times, and other fonts that stomped out the previously installed MS fonts. Office 2016 for Mac fonts are within each MS application bundle, and not installed in System locations.

You options are to live with the High Sierra version of the fonts, or uninstall Office 2008/2011 per Microsoft instructions, and then reinstall/update to last respective Office updates. Of course, the next release of High Sierra will just stomp on these MS fonts again, so it may be better to just adapt, than experience this repetitive process. I would not forego operating system updates to maintain MS font status quo.

The characters are certainly wider, so very slightly more 'bold', but the difference is much less pronounced than it was before. I have hundreds of documents which rely on the difference being much easier to spot...

Surely this is a bug - they can't just change a standard font like this... can they?

For word problems, best consult the forum devoted entire to that by its maker. Make sure you are clear about the version you are using and have installed the latest updates.

Results in Office - Microsoft Community

Same issue. On MS word in office 2011 and 2016 times displays incorrectly. Bold formatting appears as expanded text rather than bold.