Most of the info is here and here Technet, but for easy step by step, follow below:

  1. Open PowerShell
  2. Identify the disks you want to remove using:
    Get-PhysicalDisk | select FriendlyName, SerialNumber, UniqueID

    This will list all the disks and their UniqueID (which you'll use later to retire/remove
  3. Mark the disks you want to Upgrade/Remove to Retire (this will set those disks to read-only)
    Set-PhysicalDisk -UniqueID xxxxxxxxxxx –Usage Retired where xxxxxxx is the UniqueID from (2)
  4. Add the same number of disks you retired to the drive pool, either via the GUI or PowerShell.
    Add-PhysicalDisk –StoragePool $Pool -PhysicalDisks -UniqueID xxxxxxxxxxxxxx -Usage Automatic
  5. Get the Storage Pool Name:
    Get-StoragePool
  6. Repair the Storage Pool:
    Repair-VirtualDisk –FriendlyName NameOfYourStoragePool
  7. Check on the status of the repair:
    Get-StorageJob
  8. Once job is complete, you can remove the drive from the Storage Pool via the GUI or PowerShell
    Remove-PhysicalDisk –StoragePool StoragePoolName -PhysicalDisks -UniqueID xxxxxxxxxxxx