Files are files and filesystems are filesystems. You keep your files on filesystems.
NTFS and ext4 are non convertible - you cannot turn one into the other directly, in place. However you can take files from one and put them on another.
Yes, moving TBs does take time, sorry it is unbearable.
Bizarre article: "Recently, Linux-based firmware has emerged as a powerful alternative"
I have a stack of Dell OS9 switches in my computer room - they boot BSD. I have sold and set up Dell OS10 switches - they boot Debian ... on the control plane. To be fair they can run quite a few OS's on the control plane. On both, you can switch to a shell (BASH) and fiddle with Ansible and the like or you stick with the usual interface.
They are not glorified PCs! Frames and packets pass through some very fancy electronics and some very specialized memory (CAM - Content Addressable Memory) is employed for certain tasks. The manuals for these beasts run to 1500 pages.
I also have a large fleet of pfSense and VyOS routers and a Mikrotik or two and a slack handful of Fortiwotsits, oh and a Cisco thing or two and some others. pfSense is BSD and the rest are Linux. The Fortis are a bit more like modern switches with their own rather odd and twitchy way of doing things, backed up with some fancy and not so fancy hardware.
I have also played with all of the distros mentioned: Tomatoe/DD-WRT/OpenWRT and they are great for cheekying up a rather rubbish ISP provided router. They are also great for running on budget gear. They are basically superb for budget conscious consumers that are capable of reading some very decent docs. Prosumer is the term, I think.
Anyway, this article is rather odd and is basically filler. The section titled: "Case Studies and Real-World Examples" is a contender for fluff of the month.