If I understand your question correctly -
The current plan is that the "New" version (once complete) would include new and significantly improved functions/features (incl mouse support), and Users would need to purchase/download the newest version to use each newly added/improved feature. The reason for this is because we will be building this 'from the ground, up' using all the newest approaches that C3 has to offer.
Regarding Purchases: While we haven't figured out (or even discussed) the logistics yet, if possible we will offer the "New" version at a largely-discounted price to anyone who had purchased in the past.
It's also possible that depending on the complexity of adding Mouse Support, it could be backward-compatible, in which case it could be released as a free update to the earlier version of NeroEngine - at no additional cost.
This would only be possible if the existing engine code requires no further changes/revisions, as we can't spare the time to backtrack and make large-scale edits to the current NeroEngine.
*Please Note: It's still early in our design/dev process and we are a small team, so unfortunately we can't confirm ANY features until they have had some time in development and test!
These are all great questions! Please let us know if there is anything else you need.