The exact threshold that Google uses to determine duplicate content is a tricky one.
The more important question is, are you noticing a problem? Are both pages being indexed (2013 & 2014)? When you search for the 2014 model in Google, is it showing up in the search results or is it being filtered out as duplicate content? If your content isn't being indexed or isn't ranking, then you have a problem.
Panda also can be an issue, but only if a large portion of your site is duplicated. Is this model upgrade process something you apply to 1 or 2 products, or are you talking hundreds? What proportion of your site is nearly duplicate content compared to original content. If the percentage is too large, you could be at risk of Panda.
If this is only for a couple of pages, you can always just take a little time to re-write the description from last year to improve its uniqueness, and also bolster the page with unique content like user reviews and new photos & videos of your product. Once a product model is discontinued, you can also 301 redirect it to the newer models.