> (assuming that's what you mean), build the (empty) raid filesystems,
> get parity rebuilt, ...   Then shutdown, remove one of the new drives,
> replace it with the half of the old raid that still works - now you have
> two degraded raid filesystems, both usable.   Copy data.  Remove old
> raid drive, replace with the new one temporarily removed, and allow
> parity to rebuild.

IMHO just copying can be done with open computer