It seems Fusion 3 will have a lot of issues with lion according to
this
However, according to the
documentation for Fusion 4, you have to uninstall Fusion 3 before installing Fusion 4 anyways, and this does not remove the disk image file where everything from windows is stored.
So you probably should backup the windows disk image file, uninstall fusion 3, then install fusion 4, then upgrade to lion, then hope everything is still working. If something breaks, you can then uninstall fusion 4, reinstall fusion 4, and use your backup to restore windows.