Upgrading from v11, should the Indexes be rebuilt?

We've update the docs to be more explicit. Yes, a full re-index is necessary as there are field addition and changes. In the case from 11->12 you can copy the index but of course would still need an re-index of the existing. Content mentioning will not work until a re-index is done. From a major to major perspective we generally recommend rebuilding it if time allows, but in this case the v11 index will load fine but hinder functionality until the re-index is complete.