When working with complex documents, sending Symbol masters to the Symbols Page provides you with an overview and helps you stay organized.
In addition, if you choose Insert › Symbol, you will see a pop-up menu containing the Symbols in your document in an alphabetical order.
If you include a slash (/) in your Symbol’s name, Sketch will treat these as group separators. For example, two Symbols named Button/Normal and Button/Pressed will be grouped together into a submenu called Button.
You can swap Symbols using the pop-up menu in the Inspector. In the example of a pressed button and a normal button, you can see you can swap between the two with an ease. You can then use overrides to customize each instance.
Aside from that, any layer can be swapped with a Symbol. Control-click the layer you want to replace, and choose your Symbol via the Replace With option in the contextual menu.