Can't upgrade the system because of held back packages

[Larian1975]

Description: Ubuntu 14.04.3 LTS
Release: 14.04

I expected to be able to upgrade my system to the 15.04 release.

apt-get upgrade reports that there are several packages that have been held back
gcc-4.9-base
gcc-4.9-base:i386
lib32gcc1
libgcc1
libgcc1:i386
linux-headers-generic
linux-image-generic-lts-utopic

Can't even do a partial upgrade. Attempting to do so is what launched this bug report with a complaint of not being able to install ubuntu-desktop.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.16.0-57.77~14.04.1-generic 3.16.7-ckt20
Uname: Linux 3.16.0-57-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sun Jan 24 11:59:48 2016
InstallationDate: Installed on 2014-06-10 (592 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-01-20 (4 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update.manager.release.upgrades: 2016-01-17T11:00:19.103252