The upgrade path is strictly limited to one version at a time, i.e. 7.04 to
7.10 to 8.04 to 8.10 - well, you get the picture, too tedious to
contemplate. It's also prone to failure.

For best results a fresh install is recommended, with root (/) and home
(home) on separate partitions so in future upgrades none of your data (but
all of your programmes unfortunately) will be lost.