No exactly true. It is possible to add new disks as separate raid group(s), in which case they will be fully utilized. It is not best pratices, but it is doable.
You'll want to add the new disks to the aggregate using the command line rather than FilerView because you can use the -g <raid group name> flag to create a raid group of just the 600GB drives. Regardless of whether you add them to the existing aggregates or create new ones, be sure to maintain 450 and 600GB spare drives. You can use the 600GB drives as spares for both sizes, but once they are downsized to 450GB, it's very difficult to return them to 600GB
You might want to look at SATA shelves as well -- there's actually a good chance that you'd be hard-pressed to saturate (4) shelves of FC disk through a 2020.