Mirrordisk supports multiple mirrors (primary & two secondaries). Here is what I would try:
vgextend the new array into the volume group and lvextend the third mirror into each logical volume. Use the physical disk parameter to make sure the third mirror establishes on the platter you expect. The old Hitachi should now be primary, the internals should be the first secondary, and the new Hitachi should be the second secondary.
lvreduce the internal drives from the logical volumes. Again, use the physical disk parameter to remove the appropriate internal disk from the mirrorset. My thinking here is that the new Hitachi drives should (??) become the first secondary at this point. Then lvextend the internal disks back into the mirror set. Hopefully they become the second secondary. The lvdisplay -v command should show the mirror relationships at this point.
lvreduce the old Hitachi drives from the group. The new Hitachi drives should (??) become primary and the internals should promote to the first seconday (hopefully). If all is well, vgreduce the old hitachi drives from the volume group.
As you can see, this is sort of theoretical. But I think it is worth a try where you are never in a situation with less than two drives attached to the data. If this doesn't work from a what's primary and what's secondary point of view, then you have no option than to break the mirroring down to the point where the new Hitachi's are the only drives in the logical volume prior to mirroring in the internal secondaries again.
Godd luck.
