You will keep the height adjustable suspension, just loose the lowest setting.
The LX uses springs that are softer than the 100 series Land Cruiser. When you lower the suspension to the lowest setting on the LX, you're basically turning off the hydraulics and sitting on the springs alone; a good 1.5"-2" lower than a comparable 100 series Land Cruiser is sitting on it's springs.
By replacing the LX springs with LC ones, when you set the suspension on 'low' (once again, basically turning off the hydraulics almost completely) the resting state will be the same as a 100 series Land Cruiser.
You will still retain the ability to raise up to the 'high' setting, though.
There was only ever one height for the LC, I'm pretty sure. He might be thinking of the difference in height between the left side and the right side, which are two different springs.
Once again, I'm sure someone here has some used springs they will sell you cheaper than new.
If you do want to go new, PM Cruiserdan (resident Toyota parts guy) and get a significant discount.