Transitions - "installing xserver-xorg-core would break existing software"
The message "installing xserver-xorg-core would break existing software" is expected (and normal) when we are in the middle of a transition to a new X.org release. Often times you simply need to wait a day or two and try upgrading, and everything should be back to normal.
The message is sometimes encountered if your mirror is out of date. Switching to a more current mirror can work around the problem in this case.
In some cases your upgrade can be blocked if you have unsupported video drivers installed. For instance, you may have installed the nvidia or fglrx proprietary driver, and there may not yet be a version that works with the new X server; in that case, either wait until a new version of the driver becomes available, or uninstall it using the Hardware Drivers configuration tool and perform the upgrade using the FOSS drivers (and then re-install the proprietary drivers when they become available). Or, you may have some obsolete drivers installed; theoretically the system should automatically uninstall them, but if it doesn't for some reason, you can force the upgrade (apt-get install -f) or manually uninstall the old video drivers and re-try the upgrade.