Disk upgrade ordering will be available in Q2. You cannot change the RAM in an appliance.
Two scenario question;  1: If I order a VxRail 120 with only 128GB RAM and want to upgrade those nodes later to 192GB RAM, am I able to ? 2: If I order a VxRail 120 with a smaller disk pack, can I add more disk later ?
VxRail Hardware Upgrades