jQuery 2.0 arrived in April with the same API as 1.9 and the IE6–8 compatibility code deleted. The 1.x branch continues in parallel — 1.10 shipped the same features — so this is not an upgrade decision, it is a decision about which browsers the page has to serve.
<!--[if lt IE 9]>
<script src="/js/vendor/jquery-1.10.2.min.js"></script>
<![endif]-->
<!--[if gte IE 9]><!-->
<script src="/js/vendor/jquery-2.0.3.min.js"></script>
<!--<![endif]-->The odd second comment is the downlevel-revealed form, needed because IE10 dropped conditional comments and would otherwise skip the script entirely. Since the API matches, plugin code does not have to fork — which is the point of keeping the version numbers parallel. What the split actually saves is around ten kilobytes minified and four or five gzipped, which is not much, so the honest reading is that it buys two cache entries and a second request to save very little. If IE8 is still on the support list, ship 1.10 to everyone and revisit when it comes off. The reason 2.0 matters is not its size; it is that the 2.x branch is no longer constrained by IE8 in what it can add next.