Hi,
in most of the cases the old CHS-geometry isn't used anymore (it can handle only 8.4 GB btw), so access to the old/new partitions should be possible to both the old and the new versions. There is one exception: some bootloaders do have problems with changing the geometry, because they need e.g. some additional fixed spaces (e.g. the sectors directly behind the mbr), hardcoded values etc.
Regards, Michael Fritscher