Data Point: I had an OC 1-5 that was converted to a NC 1-4 by the UK CS phone number last September. At the time, the CS rep promised me the NCs could be upgraded and extended like the OCs. I called last week to extend my NC as I'd been promised, having followed the discussion on this thread and the original thread. I was told they couldn't extend it as the conversion was considered an extension. I explained my previous conversation and what I'd been told. I even asked if they could cancel my old partial certificate and give me a new one. They said no - all they could do was open a "case". I felt this was far more 'will not because it's against policy' than 'the system doesn't allow us to'.
About 10 days later and after me chasing them again, they have extended my partial certificate to 1 year ahead from today. They reiterated that this was a special concession because they had listened to my original conversation where I'd been misinformed, and that the conversion of OC to NC was considered an extension and that no further extensions were allowed. In fairness to the person I originally spoke to, it was last September when it seemed like Bonvoy Central hadn't fully worked out how to handle the OCs and their converted NC equivalents.
So I got what I wanted and was originally promised, but it took a fair amount of effort on my part to convince them to do it.