NetApp has 2 basic variants of its filer operating system.   For Data ONTAP 7, the original Data ONTAP is called “G” as in “Data ONTAP 7G”.  For ONTAP 8, they say “7-Mode” as in “Data ONTAP 8.0 7-Mode”.  7G or 8 7-Mode are the same thing, the original Data ONTAP for filers. In addition, the NetApp sales people have figured out that customers are reluctant to upgrade to 8, so they coined the term “7-Mode” to make it sound friendlier somehow.

C-Mode or Cluster Mode is the latest Data ONTAP operating system.
In 2003, NetApp acquired Spinnaker Networks, who made appliances running SpinFS, a distributed file system.   Subsequently, NetApp made separate clustered versions of Data ONTAP that were based on SpinFS and had little to do with “real” Data ONTAP.  These include Data ONTAP 7GX (“X” = cluster) and Data ONTAP 8 cluster-mode.

Customers have to decide at install time if they are going to run 7-mode or cluster-mode.  They are 2 completely different install images.  There is no “converting” between 7-mode and cluster-mode, you have to do a complete re-install and copy all data.