grow MULTIPATH RAID

Hello Experts,

Please help me to grow MULTIPATH RAID.  
My problem:
I've Linux box which have MULTIPATH RAID created on them. Now we need to remove
those old LUN and move data onto new LUN presented to those boxes.

My initially thought was to grow the  current MULTIPATH RAID (/dev/md0) and 
make old LUN as spare and then  failed and then remove(Is this a right
approach?).  When I’m try adding new LUNs to it, they are getting added as spare
( as shown below).  And when I try to grow , it refuse to do so.  Below are the
steps that I performed and error message.(Cannot reshaped!!) 
( I read somehwere  that we can’t grow MULTIPATH RAID, if it is TRUE, then what
would be suggested solution to move data from old LUNs to new LUNs)
Please help
--------------------------------------------------
 
# mdadm --detail /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Thu Mar 24 13:54:41 2011
     Raid Level : multipath
     Array Size : 10486016 (10.00 GiB 10.74 GB)
   Raid Devices : 2
  Total Devices : 4
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Mar 24 13:54:41 2011
          State : clean
 Active Devices : 2
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 2

           UUID : xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
         Events : 0.3

    Number   Major   Minor   RaidDevice State
       0      65        0        0      active sync   /dev/sdq
       1       8       96        1      active sync   /dev/sdg
--------------------------------------------------------------------
# mdadm /dev/md0 --add /dev/sdr /dev/sdh
mdadm: added /dev/sdr
mdadm: re-added /dev/sdh

--------------------------------------------------------------------
# mdadm --detail /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Thu Mar 24 13:54:41 2011
     Raid Level : multipath
     Array Size : 10486016 (10.00 GiB 10.74 GB)
   Raid Devices : 2
  Total Devices : 4
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Mar 24 13:54:41 2011
          State : clean
 Active Devices : 2
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 2

           UUID : xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
         Events : 0.3

    Number   Major   Minor   RaidDevice State
       0      65        0        0      active sync   /dev/sdq
       1       8       96        1      active sync   /dev/sdg

       2      65       16        -      spare   /dev/sdr
       3       8      112        -      spare   /dev/sdh

--------------------------------------------------------------------
# mdadm --grow /dev/md0 --raid-disks=4
mdadm: multipath array /dev/md0 cannot be reshaped.



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