If your subscription is still valid, your old Mapbox and Nearmap credits will remain applied to your license after upgrading to V3. You will not lose them and they will not expire.
However, Mapbox and Nearmap credits can not be used to import aerial imagery with V3. Once you upgrade to V3, you need to convert your old imagery credits to the new PlaceMaker credits. After converting, the new credits can be used to download the same amount of imagery as the previous credits entitled you to download. However, you can also choose to use your newly converted credits for other types of data such as buildings, 3D Mesh, and SmartSite.