There is no obvious means of using
".be/version"/"libbe.bugdir.TREE_VERSION_STRING".  In the past I've
worked around this by keeping all the disk-reading backwards
compatible (e.g. homemade mapfile -> YAML, the "From" hack in
libbe.comment.Comment.load_settings, possibly others).  However, this
is not the road to easily maintainable code.

Most projects only need to maintain backwards compatibility with the
last few versions of their disk cache, to allow users an easy upgrade
path.  The difficulties come with "be diff", which must be able to
read _every_ disk-image of the bugdir ever committed into something
comparible with the current cutting edge.  This makes sweeping changes
very difficult.  VCSs themselves avoid this by never showing their
disk-cache to another program, but we've shown ours to the VCS, and
it's difficult (or impossible, depending on the VCS) to change history
to match the current format.