Watch My Domains SED will normally overwrite the previous whois data in the database with the new one when a new lookup is performed. However, it is possible to preserve the old data. To do this add the following to the config.php file.
$config_data['enable_data_history'] = "whois";
Make sure that you have the latest build.
After you do this, the whois data will be logged along with the time-stamp to the history data table in the database. Both registrar and registry whois are saved. You can then use the Details tab in the lower pane to access the old whois records.