This does not require a outage all you need to do is add in all the new disk to the disk group and then remove the old disk from the disk group. I would recommend you do this during a time when there is low IO activity.
I had the same issue several months back we need to move to EMC from IBM so we added all the disk from EMC making sure to match the total size of the diskgroup as well as taking into account mirror if you are using normal redundancy. Then we removed the old IBM disk. This was all done online and during the evening hours the work took about 5 hours to complete.