Hi, welcome to Sitepoint
This article goes over some of the differences between those versions of bootstrap but bootstrap 3 is really a departure from previous versions and not really something that you can easily upgrade to. It has a different methodology (mobile first out of the box) and is really something you'd use on new projects rather than trying to upgrade an old or existing one.
From what I can gather most of the component changes are to the 'flat' design and lose most of their border-radius and gradients etc. I'm not sure the size of components has changed that much. However, I haven't actually used bootstrap 3 yet but have just completed about 150 pages in bootstrap 2 so upgrading for me would be out of the question and slightly pointless.
Sorry, I can't offer more specific help.