yesterday evening i did a sudo apt-get update and sudo apt-get upgrade, no problem at all
if i can remember correctly (but don't know for sure) i only got a few minor messages about rights and some files it couldn't find and at the end about some double repo entry,
but i think its supposed to work this way, Lmce only lets you know this stuff, i think these messages are not error-messages by default.
I now have a working Core and among other great improvements, can tag media using the TVDB and IMDB
thanks all for making the update process a bit clearer.
and YES LinuxMCE is much more intelligence than it shows at first glance