How secure is it to upgrade bitcoind?
Although certain releases perform database upgrades, the upgrade is handled by the bitcoind binary - updating a node only requires changing the binary, regardless of whether you downloaded a prebuilt one or compiled it from source.

If any additional changes to the bitcoin data are required, the updated bitcoind will perform them when it starts.

If you are downloading a prebuilt binary, make sure you get it from a reliable source (such as bitcoin.org) and verify the signatures to make sure it has not been tampered with.