In ConvertKit for Caldera Forms 1.0.4 or later, you can change the Converkit sequence ID or Convertkit form ID that you subscribe users to with filters as well as UI options.
You can change the Convertkit form ID using the cf_convertkit_form_id filter:
You can also change the Convertkit sequence ID using the cf_converkit_sequence_id filter:
Where Does This Code Go?
When using WordPress hooks to customize Caldera Forms or other plugins you should not modify the plugin files, or you will lose your changes when you update the plugin. Instead you should create a small plugin to hold the custom code. It's easy, learn how here.
Technically you can add the custom code to your theme's functions.php, but then you will not be able to change your theme and keep these customizations.