you can export and import to rx7640, is it running hp-ux 11.31?
first, in rp7440 run hp-ux 11.11, check the disk layout version:
# fstyp -v /dev/vgxx/lvxx
hp-ux 11.11 support up to version 4, and hp-ux 11.31 support version 4 to 7. so if disk layout in rp7440 is 4, you can mount these disk to rx7640, but if not, you must upgrade disk layout at rp7440:
# vxupgrade -n 4 /mount_point