*cough*3rd edition Raiders*cough*
I've done this conversion myself - the old Raider and Ravager model are highly similar, just like the new ones. If you have extra lances or dissies for the Raiders you should also have the extra gun poles they mount on (if you're using new model lances/dissies then just look at the mount on the old Raiders and realize it's a super easy conversion) after that all you need to do is add side shields.
I've used old Raider platforms carved up to make the shields, I've also done it by just adding broad blades across the side in a shield design to give the "heavier armored" look of the Ravager.
It's a simple conversion that can be done a handful of ways and look excellent - the big required elements are the side armor and the three gun mounts, and you could make the gun mounts out of spare sprue if you wanted, and the side shields out of blade or sail attatchments, or extra side platforms very easily.
Hope that helps!