Are questions that ask for instructions on how to upgrade to an Alpha release off-topic if they don't ask for any information about how to use the Alpha release etc, but only ask how in the current release to upgrade to one? That is problems with the upgrade, and not just where to download a new image.
The off-topic rule about future versions of Ubuntu was designed to stop questions that are either unanswerable —because a decision hadn't been made yet— or things that needed reporting on Launchpad. Both have valid exceptions, where having an open and answered question here provides more benefit than harm.
If a user upgrades and things break (during or after upgrade), they likely need to be on Launchpad reporting a bug in some form but I wouldn't automatically start closing things because they can be construed as bugs. Look into it. If there's a LP registered bug and/or a workaround, you can answer it. Or close it against a master question that deals with the same issue.
That's much more useful to everybody involved. If duplicate questions come in, we have something to close them against and people will find those (one way or another) before they add theirs. Search Meta for bugs and you'll see plenty of arguments for and against this.
If their question is asking how to do something, that doesn't really fall into either of the original reasons. Closing something just because it's about a development release is not within the spirit of the close reason.