We would like to alter the partition without losing data. Here is the sketch of the plan. I am wondering if it is doable using mdadm; and if so, please kindly point some reference on how to carry out the following steps.

For RAID 10, there are four disks used.

(1 2)(3 4)

The idea is to work on 1 and 3 while keeping 2 and 4 as backup.

1: break RAID 10 into two arrays of RAID 0 => (1 3) (2 4)    (how to?)
2: re-partition and format (1 3)
3: copy data from (2 4) to (1 3)
4: re-partition and format (2 4) exactly as (1 3) 
5: join (2 4) with (1 3) to form RAID 10    (how to?)

Does it sound doable? Thanks a lot!

Add:

It looks like this guy (drumfile) is doing something similar, but lack of enough detail.