Hello Zoid, Yes you can change the raid level with using your extra disk.  You can also do it online as well.  I would also make sure that you have a backup before you start just in case anything should happen you can restore your data.   Please let us know if you have any other questions.
Hi, I have on a md3200i 12 x 1 Tb disk, with this layout: 11 disk forming a raid5 with one hot spare. I was adviced that raid5 with 11 disks is not a good idea, because of the high probability of failure during rebuild, so I ask: is it possible to change the setup to a raid6 with the hot-spare without trashing the data? Can it be done online?  I wouldn't mind not having a hot spare if I can survive 2 disk failures.
Upgrading raid5 to raid6