HMD has confirmed that the current Nokia smartphones that are running Android OS out of the box will get next Android updates, the current models will get Android O that probably will be rolled out in 2017 or early 2018.
It was already confirmed that the Nokia 3, Nokia 5 and the Nokia 6 will get Android Oreo i.e. Android 8.0 update most likely within few months. But that is not all, the company will be rolling out Android P updates as well.
Since all the Nokia Smartphones powered by Android OS are running (near) stock android, it is easier for companies to roll out updates in case there is one. Since the OS is not altered/customized much, the company can almost immediately pass on the update.
The company also mentioned that they will unlock boot loaders from their smartphones, that way once the official updates are stopped users can still use custom ROM. Also HMD is providing updates up to Android P that would mean almost the handsets full life cycle if handled properly.