I did the downgrade in this way.
First of all I upgraded the old system by using "aptitude update" and "aptitude upgrade". So that the version was 10.04.4.
Then I archived with TAR utility the whole disk of the old system. I did it while the old system was online and stored the resulting archive on the old system.
Then I created a new base virtual machine with the webhoster utilites. At this point I had the problem to be able to overwrite existing files which were in use. So I needed to have a sort of Live Cd ( like Jeff suggested above )
Not being able to boot from a live CD, I used the webhoster rescue system to have a similar effect. When you boot into this, you can mount read and write the actual system.
So I expanded with TAR the archive in a /NEW directory, and then I copied the varioust dir like var, usr, ... etc.. I renamed the existing with the suffix OLD. Then I rebooted, hoping that all was ok. There we some problems, however.
On the first reboot, the drive could not be mounted. The reason was that on the new system the drive was formatted with EXT4, while in /etc/fstab there were the settings of the old system, which was EXT3. So I rebooted again in the rescue system and take the original fstab in the new system.
At the next reboot, drives were mounted, however the system could not get an IP address.
The reason was that, since I had completely overwritten the /etc dir also network config. I did not know how to reconfigure, so I grep to find in the text files eth0. I found many entries, but the one to fix was the one referring to the MAC address. Once restored to the actual MAC address of the interface of the new system network was working again.
OK, now I tested if apache and mysql were working. None of them was working.
It turned out to be a problem of permissions. I am not sure about this, may be I forgot to add the p prefix to TAR ( which should preserve permissions )
In order to avoid setting permission manually, I used this time RSYNC to copy again the files from the old server. It seemed that RSYNC restored the permission, because after this I could start my blog.
Now another error is logged related to log rotating. Sure it's again a problem of permission.
At the end, sure I think my method was not very professional, that was the reason I asked here :)