You should not need to fail the drives, just pull a live drive, replace with one of the larger drives, let rebuild complete, go on to the next drive.
Two ways to utilize the added space...
The addition space would be usable as a separate OS volume, which would be created within Disk Manger.
Or with a third party partition manger (Acronis, Paragon etc) you could resize your present OS volume to use the addition space presented by the new drives. If Win server 2008 you could do this Disk Manager without a 3rd party software, within it's limitations.
As a precaution, I would backup first, run chkdsk on the array before resizing; never had it go wrong, but it pay to be cautious.
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Chernobyl disaster..a must see pictorial