So if I understand correctly, you're talking about a single organization and your worry is that not all licenses were bought at the same time. Thanks to the co-termination system, your organization will have come up with a single expiration date even though those licenses weren't all bought at the same time.
For the conversion it's only that date that is important. Because that date will be the expiration date for the (per-device) licenses generated during the conversion. The conversion procedure totally ignores what the original duration of the bought licenses was.
So before AND after the conversion, everything will be expiring at the moment you labeled "Expiry date for all".
After the conversion, any licenses that you buy for that organization will now be valid for their actual duration, there will be no automatic co-terminating anymore.