Switching from JBOD to SHR.

I currently have a 4 bay Synology NAS, with 2x8TB drives configured as JBOD. These drives are almost at capacity. I understand this configuration is "bad", for want of a better word, and that SHR would be better. Am i correct in thinking it's not possible for me to just convert from JBOD to SHR?

If i were to purchase a 16TB drive, would I be able to set this up as a new, separate, SHR volume, and then copy over the data from the 2x8TB drives?

Following this, my plan is to wipe the 8TB drives and add them to the SHR volume. Is this a smart idea? From looking at the Synology RAID calculator, this would leave me with 8TB of unusued space. Is is better to have matching drive sizes?

I'm very new to this, so please be gentle!