Version 2.0 introduced the plugin architecture that is used for almost all the customizable functionality of Smarty. This includes:
With the exception of resources, backwards compatibility with the old
way of registering handler functions via register_* API is preserved.
If you did not use the API but instead modified the class variables
$custom_mods, and other ones directly, then you will
need to adjust your scripts to either use the API or convert your custom
functionality into plugins.