Hi Michelle Couden,
I think you have to make a distinction here between the Feature Layers, the Web Map and the Web Mapping Application.
It is the Feature Layers that contains the data for a certain year, say 2020.
You combine these Feature Layers together with a basemap layer into a Web Map. And this Web Map will be presented in a Web Mapping Application.
It is the app that contains all the functionality (zoom in, zoom out, search, layer control, legend, etc.)
The only thing you will have to do early next year is to publish the Feature Layers for 2021 and update the Web Map with these new layers (removing the 2020 ones). In this way your Web Mapping Application will stay entirely as it is, automatically showing the new 2021 data. And you have a backup of your 2020 data as long as you do not delete these Feature Layers.
Is this an answer to your question?
BR,
Egge-Jan