OK, you are right.. only customization of drive letter is allowed.

Now to answer your question, the one and only formal way is to delete the pool by archiving all the Persistent Disks (User Data Disk), then  create a pool with same name (but without SDD).

Further, select all the archived UDD and then click, "recreate". All the new desktops will be recreated with new spec,

Another way (not officially validated/ not sure of its impacts)

pae-SVIVmPersistentDiskSpec entry will be there at ADAM database OU=Server Groups (VMware KB: Connecting to the View ADAM Database)

In this one entry will be there for system disposable which has a parameter "DiskUsage=SystemDisposable"

Upron removing the corresponding entry *may* end up in not configuring the SSD for further provisioning.