I am saying that you may think that is what happened but it is not likely
the case because Ubuntu does not work that way. You cannot switch versions
at the login screen because the kernel is part of the version (10.04 and
10.10 use different kernels) and those are loaded before the login screen,
from grub. In order to switch you would have to reboot, not just log out and
in. I would be interested in what it says when you type this in a terminal
in each case:

lsb_release -a