Sounds sketchy, in a supported-but-not-really kind of way. I have trouble picturing OSRF advocating such a path. I know the package GBPs and therefore supported versions are locked at bloom-time, but I wonder if it would be possible to spin an experimental farm that builds Hydro binaries for Saucy and Trusty based on the Hydro/Raring GBPs. Some packages would fail, but I bet it would still be enough to pretty useful as a transitional tool. Could it even just be done as a build of all the existing Raring sourcedebs for Trusty?