Hello,
for the case there should be a power outage, will the entire blockchain be downloaded again or only the changes (since the outage)?
Thank you!
Most commonly a power outage results in irreversible corruption of data bases relevant to blockchain copy on you disk thus IBD will be inevitable fact of life if yuo don't have the backup.
I have backup 2TB SSD disk (which I synch on regular monthly base) for such cases. Thus should the outrage happens I will copy the contain of disk to SSD which is attached to my bitcoin node.