In-place upgrade from 7-Mode to C-Mode is not possible. Filer must be wiped out and cleanly reinstalled in C-Mode. If there is data to be preserved, it must be evacuated (migrated). 7MTT helps to automate this.
Correct me if I am wrong, but it seems like eventhough the FAS2552 is in 7-Mode, the firmware in the filer also has CDOT image in it and all you need to do is change "boot mode".
That's correct. Except it is not "firmware" but "software". As you currently have 8.2 and want to go to 8.3, it makes sense to install 8.3 first (which is additional step KB article explicitly mentions) and then do initialization (boot menu option 4). It is simply faster than doing update later. Make sure to install the same version you have on existing cluster if you intend to join it.
And of course you need to perform this procedure on both controllers independently. Note also that if you want to use Advanced Disk Partitioning (normally default wit 8.3 on FAS2500) you should remove ownership of all disks before starting initialization.
Forgive my ignorance, I am not seeing that upgrade step in the KB article (i cannot access official KB article only the link you sent me (google), do you think you can send me the official KB article because I am unsure of the upgrade step.
I am assuming from my 8.2 7-Mode system i can download the 8.3 from NetApp, and install as per instructions eventhough i am operating in 7-Mode. Then after change boot env options? Then after initialise disks?
You cannot install 8.3 using google link. This link explains how to reinitialize controller in C-Mode using already installed Data ONTAP version. You can then upgrade it of course from within Data ONTAP.