Comment 2 for bug 1576531

This causes all package upgrades after squid3 to fail and the upgrade to be reported as successful with errors.
The system now boots up as 16.04, but the rest of the packages still had to be installed to "truly" finish the release upgrade.

This is easy enough to resolve by uninstalling the squid3 package and installing the squid package, since 16.04 has a newer version than 15.10 did anyway.
Just be sure you don't lose your configuration (make a backup).

apt-get update
apt-get remove squid3
apt-get install squid
apt-get upgrade