With HCI typically you can set it up & scale/downsize infrastructure easier compare to CI.
I would make an analogy with homes. There are handmade costly and stylish houses, and on your land, as with Lego you can do whatever you want to, but it means, you need to do that whatever you been thinking of.
And there are excellent and comfortable condos with many amenities in it, but not to go beyond that.
So with FlexPod, you can have FAS/AFF, SolidFier, E-Series, blades or rack servers, many OS, Hypervisors and applications and it is not limited to existing architectures & designs.
With HCI you can quickly start, scale/downsize, but it has only three types of computing nodes & 3 types of storage nodes, and it is only VMware.
If you have invested quite a bit in CI, I don't see the need "replace" or "convert" to HCI... the latter is a matter of having a pre-assembled compute+storage+network coupled with an "easy" setup process... maybe for your next innovation iteration you could consider HCI and have a more closer look ! 🙂
I don't know if I'm too late but I'll give it a shot anyway. From a general perspective of HCI compared to converged infrastructures, I would argue that HCI systems are made of more closely related components tied together with a virtualization layer touching compute, storage and networking. Plug, on top of this, in order to make the whole thing feel like a black box, an HCI platform will come with a management pane offering a single point of management and a holistic view of the components, their usage and providing the required functions to provision or decommission ressources as needed. Also, ideally, most of the interaction with it should be doable via APIs. The latter is the key to be able to build a managed infrastructures and make it operate like cloud providers.
Now, you tell me if you get a better understanding with my explanation.