for the case there should be a power outage, will the entire blockchain be downloaded again or only the changes (since the outage)?
This only always applies in case of corruption if your blockchain is "
pruned".
That's because whether the corruption is only your chainstate or block index, the pruned blocks needs to be re-downloaded and verified to rebuild those data.
Otherwise, most of the time, you'll only need
--reindex-chainstate or
--reindex depending on the error in your "
debug.log" file.
Those wont download the blockchain but build the target database from your existing blockchain.
Don't blindly restart IBD or
--reindex without consulting your logs first since it could save you a lot of time and/or bandwidth.