Ditto the other comment. I've done this many times. Replace one disk at a time. Watch the rebuild and when it's done and the array is healthy again, replace the next one.
When you've replaced all of them you can do an array expansion to grow your logical array and use the rest of the space, or if you like create a new logical array using the new space.
The choice is yours, although online expansion requires a battery backed cache module.
You can speed things up a little bit by modifying the priority of rebuild and expansion tasks to higher priority, which will slow down normal access a little. If you want to keep the OS running as best it can, set the rebuild to low priority, but rebuilding 300GB of data will take a while at low priority. I think at high priority I've seen 300GB rebuild in about 90 minutes.
