You have 2 options...

If you install the EZPopulate mod you can download a complete file of your products and then change the model numbers - import it again - and let EZPopulate handle the mass update.

OR you can set up your manufacturers and use that field on the product display without using the model number by assigning all the products to their manufacturers ( the advantage to this is that if you use the manufacturers sidebox and do it this way people will be able to "browse" your products via manucturer).

Hope that helps!