1. You can't mix SAS and SATA in the same array, so replacing a 300GB SAS disk with a 1TB SATA will NOT work. 2. IF you were to replace the 300GB SAS disks with 1TB SAS disks, you could reconfigure your array to include all the space on the disks, however ... 3. If you reconfigure your 1.5TB VD to a 5TB VD, you MUST have already installed the OS in UEFI boot mode. If you currently have your system in BIOS boot mode, then you are either limited to a 2TB VD or will be required to reinstall the OS in UEFI mode.  4. Reconfiguring a 6-disk RAID 5 to RAID 6 requires adding a 7th disk, regardless of the disk space involved.
We currently have a Dell PowerEdge R610 (PERC H700 integrated controller) with 6x 300GB SAS drives in a RAID5 configuration. We want to replace these with 6x 1TB drives in a RAID6 configuration. We need to do this with as little downtime as possible (preferrably none). Is it possible to swap each small disk out with the larger disk, and then do a live reconfiguration from a 5+1 RAID5 to a 4+2 RAID6 setup? The documentation states that you need to add a new disk to reconfigure from RAID5 to RAID6 since RAID6 provides a smaller virtual disk, however since the new disks are significantly larger, it should theoretically be possible to do the reconfiguration without adding a new disk while maintaining/increasing capacity. Whether the controller actually supports that is my question.
PERC H700 reconfigure RAID5 to RAID6