Yes you can upgrade via the CD but the chances of it working any
better than the on-line upgrade aren't any different. Upgrades have
always been troublesome ( and that goes for *any* OS, not just Linux )
though there are users who have done it with no trouble - several
times too. I suspect it all depends on exactly what's been installed
and whether default settings have been changed, either because they
needed to or part of the process of learning and although the changes
aren't causing problems now they can lead to a failure to upgrade
cleanly.

Overall the best advice appears to be always to do a clean install
with the attendant updating / installation of software. It can help
having a separate Home folder to prevent having to save data, but we
all should have backups anyway so it shouldn't be a real problem if
the Home folder is erased ( or the HD dies ! ). On a reasonably fast
broadband connection the process shouldn't be too onerous but not a
good idea to start if you can't allocate enough time to it - the day
before an important piece of work needs to be finished is not the time


You can try the update by all means ( whether CD or on-line ), but be
prepared for it to fail and not be in a panic about data loss if it
does :-)

Just a final point - before attempting an upgrade make sure that all
updates for the current version have been done.