One of the design goals for Screenly is that it should require little to no manual maintenance. As a result, we have designed Screenly such that it automatically updates itself every night. The only requirement is that the node can connect to the internet.
Because of this, there is no need for you to replace your nodes every time we release an update.
If you are installing a new screen, you should still download and use the latest image, as this will speed up the installation process significantly.
*If you are running a version of Screenly from early 2018 or before and have not upgraded to Screenly v2, then review this article: How do I upgrade to Screenly v2