Having overlap for at least one revision allows seamless in-place upgrade.
Having zero does not.
No Dan, it does not allow a seamless in-place upgrade, unless you only ever upgrade once.
If SA version N+1 supports commands from N, and your config file is in version N syntax, you can upgrade seamlessly, but as soon as N+2 comes out, you won't be able to upgrade seamlessly anymore.
Right, but if N+1 supports both syntaxes, you can upgrade to N+1, then change your config files at your leisure before upgradign to N+2. If N doesn't support the new syntax, and N+1 doesn't support the old syntax, your only choice is to take down the system, change the files, then bring it up with the new version.
