Removing old roof vents units when getting a new ridge vent?

We are getting a new roof on our 1950’s ranch-style home. Our goalis to try everything we can to stop the persistent ice damns we have on ourroof in Michigan’s long, snowy winters. But we are on a limited retiree’sbudget too so we can’t really afford to add the big, expensive fan options inour attic’s crawl space. We already have two gable vents on the east and west side ofour home. Three years ago we added 6 more inch's of blown in insulation to total 12”.This summer we finished adding air vents to the sofit and madesure that the insulation is not blocking the airflow. We are getting a long ridgevent with the new roof to add more air circulation. My question is this: With awhole house ridge vent should we remove the old roof venting units (circa 1992)that were put on in the last roof replacement?
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