Old site posts broken after domain update

It seems like you are having permalinks issue which is usually occur due to .htaccess file. Can you please go to the Settings -> Permalinks and press update. Make sure your .htaccess file is writeable, usually wordpress show warning on that page if file is not writeable. If its not writeable you will get code on that page at the bottom which you have to place manually in .htaccess file.

.htaccess file can be found in your wordpress folder where wp-config.php file resides.