My ex business partner patented a product in 2004 and failed to pay the maintenance fee's. I had an exclusive licensing rights on the product. Since then i have made several changes to it to make it more diverse and safe. Can i patent the product?
Patents are applied for the benefit of the true inventor or the inventor's assignee. If you didn't invent it you can't patent it.
The expiration of the original patent has zero bearing on the patentablity of your improvements. If your improvements are novel and non-obvious considering every that has been previously, including the patent disclosure, you might get a patent. This is based on the disclosure in the published application/patent regardless of its status as an in-force patent.
However, assuming a product based on the improved version infringes the claims of the original, the original would not be an impediment to selling your new version since the old patent is not in force.