Yes, you should be able to replace the drive.
Some external drive manufacturers, though, "hard-code" the drive capacity into the enclosure's firmware -- meaning that if the original drive was 80GB and you put in a 200GB drive, you may be only able to "see" 80GB (the size of the original drive). Maxtor was known to do this with their external enclosures.
Best thing would be to contact LaCie tech support and verify that this is not the case before spending money on a new drive. I highly doubt that LaCie would do this, but, better safe than sorry...