I have always heard that you can reposition drives (with the system powered off) in an MSA without affecting data. This is essentially what happens during a data-in-place upgrade when moving between MSA platforms. The second bullet point on page 11 of https://h20195.www2.hpe.com/V2/getpdf.aspx/4AA4-6830ENW.pdf? would back that belief of where it says "The same drive order is not compulsory as the metadata with be identified by the MSA
2040/2042 controllers. Moving drives should only be done while the system is offline."
Of course, make sure you have a backup of your data.
On your second point you mention having to remove your current SSD vdisk and recreate. You also mention using the SSD's as "cache" now. So, not sure exactly how you are working now. If you are using the SSD's as read cache then you can simply mark additional SSDs as read cache for each pool (up to two SSDs per pool). If you are using the SSDs as a "tier" within the pool then you have two options. Easiest is to create new SSD vdisks and add them to each pool (ideally the vdisk would match the RAID type, and set size, of the previous SSD vdisk). Of course, you can also remove the current SSD vdisks, and recreate them with new drives included, but there will be some implications to this. You'll need to wait for the SSD vdisk(s) to be drained (VDRAIN). There will be some performance implications of the VDRAIN itself along with the fact that you temporarily don't have SSD in the pool.