Yeah, this is a tricky one. It's not like reviewing a book that remains unchanged once it has been published.
Times change, companies change, their software/platform changes.
eBay and other services have mechanisms for weighting more recent reviews higher than older ones. That's one option.
Grouping the ratings by year is another option. People can then see how companies have improved/declined over the years. The headline ratings displayed to the user could be for the most recent year.
In any case, the whole ColdFusion application could be ported to PHP and at the same time take the opportunity to improve the functionality.
Symfony or Code Igniter are good choices for the PHP framework to make the job easier. If you have to support PHP4 and/or MySQL 4, then CakePHP.
Coincidentally, I've recently ported a bunch of ColdFusion fusebox applications running on MS Access no less over to PHP/MySQL using the afore mentioned CakePHP. It's not that painful and you then no longer have to worry about running a ColdFusion server which is a big plus.