Synology NAS – upgrade “drive in place” Q

I have two Synology NAS’s. A monster 920 with 10TB across 4 drives used for all my work stuff, including running a couple of VMs.

It “replaced” my older 218j+ which is a 2 bay drive with 2×3.6TB WD drives configured in SHR-1 (so mirror). Stores lots of static data including a lot of video renders and imovie libs. It’s running out of space. Synology tell me I can buy a new 2x8TB drives and insert them 1×1 to increase the storage pool without losing any data:

This all sounds too good to be true. I *could* back it up to the big NAS except I don’t have quite enough space left to do that, and it’d be a pain to free it up (ie messing about with my stack of external HDDs). I’m going to stick with HDDs as no requirement for faster speeds as it’s just a repository really for stuff i might need and have no space for on the Mac.

Having done some research: https://www.ballicom.co.uk/not-assigned-hat3310-8t-.p1592363.html – these drives get some love and seem a decent compromise between capacity and cost.

Anyone done something similar? Any other way round that either risking it or backing up the whole shebang before adding new drives?

I’ve upgraded before by replacing one drive, let it sync, then replace the other and it worked fine but I always have a backup of the NAS anyway

Yes same a leffe states, shut it down unless they are hot swap able, pull one out replace with big drive.

Then storage manager rebuild raid.

repeat process for every drive till all completed , once all drives are done increase the size,

My 418 is limited to 15TB pools, so i now have multiple pools