"If an application is based on an earlier version of CRS, follow the migration steps described elsewhere in this guide"
Not sure about where the migration document is referring to but I can mention some major changes in CRS 10.2 which you would have to look out for while migrating. Firstly CRS 10.2 would now require Endeca with category pages and search results page are using Endeca Experience Manager. CRS now also covers the new 'Returns' functionality in commerce, RightNow Knowledge Syndication widget on product details page, and some optimizations in checkout implementation.
You need to include the module named Store.KnowledgeBase in runassembler while building your EAR. Also, as Nitin specified Returns has also been introduced as part of ATG 10.2. You need to execute the SQL corresponding to Returns and KnowledgeBase in your Production Schema.
The best approach would be to take a dump of your Publishing Data that is getting used in current application. Then install ATG 10.2 freshly through CIM and then import the data in ATG 10.2.
They have modified the path of certain Formhandlers in ATG 10.2. Also StoreRepository has been renamed to Location Repository and so on.