RAID rebuilds are a protection mechanism and are not designed for data migration. To this point, assuming your 750 GB drives are supported with your RAID adapter, the RAID group will only utilize the first 400 GB of storage on each 750 GB drive.
Remember, RAID groups typically require that all the drives in the group are of the same type, speed and size (see my previous articles for more details on this). This means that even after you go through your "migration," you'll still only have four 400 GB SATA drives worth of available storage. The remainder will be inaccessible.
The usual way to migrate data with internal server-based storage is to copy the data to another separate set of drives or temporary storage, internally or on your network or a (wince) large USB hard disk drive (the wince means "not recommended but can suffice if you're in a pinch"), install the new drives and copy everything back, remembering to preserve all file attributes such as permissions and access dates, etc. You can use a tool such as Robocopy.
It's headaches like these where a consolidated mid-tier storage solution, such as those from EMC, IBM and HP, comes in handy. They can migrate data from one RAID group to another without any disruption and usually while the application is still accessing the volume! If your company can afford it and has a significant enough storage management requirement, I strongly suggest investigating this approach.
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