i had the same problem today
i found a util called raidreconf
i copied my old raidtab file to raidtab.old
added the new drive to the new raidtab
and ran the following command
raidreconf -o raidtab.old -n raidtab.new -m /dev/md0
heres the man page for raidreconf
http://linuxcommand.org/man_pages/raidreconf8.html