I had the same issue. Here's what I did;
-Backed up all important data from 12.10
-Booted to BT5r3 live cd and installed using entire hard drive option
-Shut down and booted to 12.04 live cd, set up partitions with the installer, completed installation process
-Reboot and GRUB appears with UBUNTU as first in list, That is 12.04. Farther down is Ubuntu 10.04.3 (I believe), that is the BT5r3 install
-Boot to 12.04 and updated to 12.10 via system updates
I know, I could have done all of it through the CLI, but using the GUI is more convenient for me. Hope this helps.



