Replacing Utility Node with Newer Generation

I'm looking for some guidance, or more specifically a good explanation of why we cannot replace an aging Gen4 utility node with a new Gen4s/t unit. We have two older mixed generation grids, composed of Gen4 utility nodes and mixed storage nodes (gen4 / 4s). The Gen4 equipment is going end of life in March and we are being told that simply replacing the utility node is not possible, so the entire grid needs to be replaced, and essentially written off.  This is obviously causing some unhappy conversations from a technical and financial perspective, especially given new storage nodes were purchased for these systems just over a year ago with the expectation that the grid as a system would remain supported.

What is it that means an entire grid is essentially tied to the Utility nodes hardware generation? From my perspective the software should not be impacted by the specific hardware version, especially as that software runs on either. I would have thought providing a new utility node, and copying over the software configuration, mcdb etc should work, so why wont it?

A detailed explanation would be very much appreciated.