if one has 2016.2 and wants 2017.2 must erase and reinstall?

The 2016.2 and 2017.2 are the build name of tested iso image of kali-rolling. Just like kali receives regular kernel upgrades, the developers of kali linux wants that updates should also be given to the iso images of kali so that users can use kali live with recent changes in the kernel.

They release 2 tested images of kali each year. Hence, the name 2017.1 & 2017.2.

However, kali receives frequent updates to packages and kernel. Actually you receive updates daily if you notice it. With these updates the images of kali becomes old so they now release weekly builds of kali so that new users can use freshly prepared kali.

Every week freshly prepared weekly image of kali is released but they are not tested and you may encounter bugs if you run kali-live.

Basically these builds are not for those who have already installed kali. It is for those who want to use kali but also want "recently" updated kernel and packages because not all people install kali. Some use it as live.

You don't have to delete your partition. Your kernel updates are handled by APT but there is no such way to update iso images like this so a new image is created and name is given by a timestamp.