Your GNOME Classic (i.e., Fallback) interface and your settings pertaining to it will almost certainly be preserved when you upgrade from Ubuntu 11.10 to Ubuntu 12.04 LTS.
Ubuntu 11.04 used GTK+2 and GNOME 2. By default it provided three session types:
Ubuntu (GNOME 2 with the GTK+2 version of Unity, which required 3D acceleration),
Ubuntu Classic (a classic GNOME 2 interface), and
Ubuntu Classic (no effects) (the same, but without any visual effects at all).
When you upgraded from Ubuntu 11.04 Natty Narwhal to Ubuntu 11.10 Oneiric Ocelot, GTK+2 was replaced with GTK+3. The Unity interface existed for GTK+3, so while it changed a lot "under the hood" (for that and other reasons), the user experience only changed slightly. However, the classic GNOME 2 interface requires GTK+2. So it was replaced with Unity 2D, which behaves very much like Unity and very little like the classic GNOME 2 interface.
As you know (since you use it), a GNOME Classic session type (which provides the GNOME Fallback interface, which is a GNOME 3 interface with a shell that feels a lot like, though by no means the same as, GNOME 2's old shell) is available in Ubuntu 11.10. It is provided by the
gnome-session-fallback package. The GNOME Classic session type is not really a logical continuation of any previous interface present in earlier Ubuntu releases, and it is community-supported rather than being supported by Canonical (that is, it is provided in the Universe component rather than the Main one). For these reasons, it would not make sense for it to be installed by default during a fresh installation or upgrade, and it is not. You had to install it yourself (by telling the package manager that you wanted that one particular package).
There are no plans to remove
gnome-session-fallback for Ubuntu 12.04 LTS Precise Pangolin, so if you use the
GNOME Classic session type in Ubuntu 11.10, there is no reason to think you will not be able to use it in Ubuntu 12.04 LTS either. Currently, this release is in alpha testing.
gnome-session-fallback can be installed in Precise systems, and if you have an Oneiric system with it installed, it remains installed when you upgrade to the Precise alpha. If
GNOME Classic is the default session type in an Oneiric system (either globally or for some user), it remains so when that system is upgraded to the Precise alpha.
When upgrading from one release to another, you should expect that the appearance and behavior of most applications will change. There will probably be changes in the GNOME Fallback interface provided in
GNOME Classic sessions. However, the changes are unlikely to be very great, because this is designed as a fallback interface; it is not where bleeding-edge development is occurring. These changes may well not even be noticeable. And whether or not they are, your settings within this interface (such as the layout you use for your panels) should remain the same when you upgrade to Ubuntu 12.04 LTS.
As there is still some time remaining before Ubuntu 12.04 LTS comes out, it is of course possible for there to be major changes. However, especially considering that this is an LTS release with an emphasis on rock-solid stability and long-term usability rather than on making major changes or introducing large new features, it seems unlikely that any of the current interfaces will be changed enormously or made unavailable.