FYI, you don't need to rebuild the indexes. You can continue running your app and the old index protocol will work seamlessly. It helps to rebuild when you upgrade in case you start using new functionality (so the indexes get computed correctly), but running an old system would work fine.
EDIT: note that you won't necessarily be able to run with indexes from multiple versions back. We'll almost certainly prune the backwards compatibility code to only a few releases to keep the codebase clean and nimble, and to keep difficult to diagnose bugs to a minimum. But it's still a very convenient feature release-to-release, as you can upgrade, and then rebuild indexes at your convenience.
EDIT: note that you won't necessarily be able to run with indexes from multiple versions back. We'll almost certainly prune the backwards compatibility code to only a few releases to keep the codebase clean and nimble, and to keep difficult to diagnose bugs to a minimum. But it's still a very convenient feature release-to-release, as you can upgrade, and then rebuild indexes at your convenience.