Module files are not backward compatible due to the change of units to mm

I've recently edited my footprint library (.mod file) with the latest version of KiCAD (ubuntu ppa adamwolf 0.201302090958+3937~16~quantal1). Later on I've realized that the units in the file have changed from deci-mills to mm but the version is still the same. The new header is the following:

PCBNEW-LibModule-V1 Sun 10 Feb 2013 01:54:09 AM CET
# encoding utf-8
Units mm

whereas the old header was like this:

PCBNEW-LibModule-V1 Thu 05 Jul 2012 12:09:59 AM CEST
# encoding utf-8
Units deci-mils

This seems to break the modules when trying to load them in an old KiCAD version (I can't check right now which version it is, but it's the one which comes with Ubuntu 11.04).

Is this a bug or is it expected behaviour?