"You will have access to all future updates."
You should have a page with all your purchased items. This will be located there. Once this is updated, it will be accessible to you again without having to pay any additional fee.
Viewing post in MZ3D - 3D Rendering Plugin for RPG Maker MZ comments
If you mean that you have to restart an entire project, then it's not necessarily needed. You will need to create all maps from scratch though. Depending on how far you are in your project, it's something to consider. Though to up the quality, I'd recommend starting from scratch as that way you go into the project with a fresh mind and a new perspective, and are less likely to have any issues lying in wait. Also helps getting that new perspective as MV and MV3D is two different worlds :)