Legacy blockchain after upgrade to SegWit Let's suppose I run a legacy (non-SegWit) node after SegWit gets activated. My blockchain doesn't store any SegWit data. What happens when I upgrade my node? Will I have to re-download the blockchain to include the missing SegWit data?
Indeed.

Bitcoin Core will, after upgrade to a SegWit-compatible version (0.13.1 and later), rewind to the latest non-SegWit block and then fetch the later blocks again.