Change following files, path given from source top level directory
configure.in: GIACLIB_MAJOR_VERSION=0 GIACLIB_MINOR_VERSION=3 GIACLIB_MICRO_VERSION=0
giac.specs, giac.specs32: version/release
debian/changelog, debianold/changelog
mkdebian
If you have links, update them, e.g.
cd ..
rm doc
ln -s giac-0.3.x/doc

etc...