I think your plan is sound. Having quality content pages with the newer images with appropriate alt and title text is a good start. Then you'll just have outrank the other images like you said.
You could also try contacting some the webmasters of other sites whose images are ranking well, and ask them to update their image with the new third generation image, if that image is just as appropriate to their content as the first generation one. For example, if their post or article is about the product in general, they could update their image of the product for you. If their post or article is about the first generation product, it wouldn't make sense to update it. This is the same as when you're link building, and you ask the author of a post or article to change the anchor text of the link they're using to your site.
Also, if your client is very interested in having people see the new images when searching for them in Google Image search now, you can run ads in Google image search. These ads use a slightly different format than both display or search ads, in that they include an image along with text alongside it. It's still pretty new, and I think a lot of advertisers haven't tried it yet, so the clicks are relatively cheap for search advertising.