Vendors sometimes have marketing messages about being able to upgrade from one storage system to the new model. A new model storage system usually means a new controller that is faster with more cache and maybe more interface connections. The upgrade may vary from just replacing the controllers (or cards within the controller) to moving the disk drives to an entirely new box. This degree of variability can result in widely different...
Being able to upgrade the storage represents a significant cost to the vendor as well. The vendor has to design and develop the upgrade capability which may be hardware as well as software. There is a non-trivial testing period required for all the different combinations of upgrades as well as the training for support and field people. Field stocking of parts for upgrade is a major cost as well.
So why have the upgrade as an capability with a new product offering? Is it such a valuable feature that vendors must have it? Do customers really do it? The truth is that an upgrade of storage systems happens rarely. Storage systems may have a 3- or 4- or 5-year useful lifespan due to technology changes, so customers usually buy new (additional) storage and retire the older systems when it makes economic sense. The storage world has changed, so upgrading storage systems by and large is an archaic concept.
When the marketing guy says that the storage system can be upgraded, there are some significant costs to enable that capability that you have to pay for. And, in reality, you're probably not going to do it. So don't ask for it or make a decision based upon that unless you really have a need for it.