I helped to delvelop a bespoke system to do this a while back with a company called Storepoint International.
Bascially the system will handle all ad size changes, re-flow changed copy into them via style sheets, images changes...etc.
It may be a bit of 'overkill' for your purposes, but if you're in a busy environment, it produces the work in about 5% of the time it would take an operator to generate the files. From a single page ad to a 100 page brochure.
If you want their details...just answer this post and I'll post the contact details
BD