The latch on the Soft rack is removable with two bolts. Just look underneath and you'll see them. The two posts at the front are also easily removable.
I would simply remove them, drill holes in the HT rack and intall them on that rack. Turn the Soft rack up side down and use it as a template to mark where to drill the holes in the HT rack. You may need to make an extension plate for the latch, but that shouldn't be too difficult.
Nice thing is, with the exception of the holes you drill in the HT rack, either rack should be easily converted back to the way they were before you made the mods.
2008 Yamaha FZ1
2009 Kawasaki KLX450R