Upgrading is already possible; just copy a new cruciforum.php over the
old, which will work fine. However, this won’t update the templates, and
it won’t update any existing generated HTML. Think of a way of handling
this. Probably the best way is to have generated HTML include the
version of cruciforum that generated it (in a meta generator tag?), and
have templates also include that information, and then have a special
“upgrade” action on cruciforum.php which looks at the templates and HTML
and then upgrades them. Add an upgrade10to11() function, an
upgrade11to12() function, etc, which know how to do each upgrade step.
This does mean that upgrades are irreversible, but, well, never mind.