MacPorts is designed so that the ports you install work on the OS version on which you installed them. Often they will continue working on later OS versions, possibly without all the functionality they would have if they were built on the newer OS. It is completely unsupportable to take software compiled for one OS version and try to run it on an earlier OS version. If you move from one OS version to another, you should reinstall all ports; see wiki:Migration.
You are correct that the instructions in wiki:howto/InstallingOlderPort only pertain to a single port. You could of course follow the same instructions for any number of ports, but MacPorts is not really designed for you to be able to install older versions of ports. If you want to backdate your entire MacPorts installation to an earlier point in time, you could of course do that by uninstalling all ports, checking out an older commit of the macports-ports repository, running portindex in it, and then installing the ports you want.
The issue tracker is for bug reports and feature requests, not technical support; for that, please ask on the macports-users mailing list.
