I see no upgrade notes for slink. For hamm, these upgrade notes reside in .../hamm/hamm/upgrade-i386/README-upgrade Usually, I just use dpkg to install whatever packages I want from the upgrade distribution. For example, I might install a newer gimp from slink. Since I wouldn't have upgraded the base packages, I learned that using dpkg prematurely can cause little problems. Upgrading from rex to bo: I EVENTUALLY HAD TO REINSTALL FROM SCRATCH. Upgrading from bo to hamm: I EVENTUALLY HAD TO REINSTALL FROM SCRATCH.
Upgrading from hamm to slink: WHAT CAN I EXPECT. With no instructions, I certainly will not use raw dpkg to add packages from slink to my current hamm distribution -- unless of course "dependencies" prevent this (do they now?).