As a temporary solution - why not just make the group they're moved in to not able to purchase that package in the store front, and instruct your members how to use the "Renew Now" button?
Mark, if programming Nexus to combine subscriptions is not doable, can't you disable purchasing the same subscription if one has an active one? Please at least add a setting per package that says "Allow duplicate purchases?" and a setting that denies purchase if another subscription from a list is chosen.
E.g., 4 subscriptions: 1 month, 3 month, 6 month, 12 month.
If you purchase 3 month subscription, not only can't you purchase a duplicate of the 3 months, you can't also purchase 1 month, 6 month, or 12 month. That way the user would have to wait until the 3 month subscription expires and decide whether to renew it manually, allow it to renew automatically, or let it expire in order to purchase a new subscription package such as 12 month.
I understand coding combined subscriptions may be hard so please let me know if the above solution works. You have told me repeatability that I'm not mentioning solutions to the issue at hand. If this is the case, please let me know what issue you are referring to because this topic was specifically made to resolve the issue I have been mentioning.