All the files that are replaced by a patch are stored under /var/sadm/patch/<patch-id>/save so the patch can be safely backed out. Newer patches will save the old files under /var/sadm/pkg/<pkg>/save/<patch-id>/undo.Z, for each package patches.
You can remove the <patchdir>/save directory provided you also remove the <patchdir>/.oldfilessaved file. Newer patches will not install a .oldfilessaved file.
Alternatively, you can install a patch w/o saving the old files by using the "-d" flag to installpatch.