Native File field data migration to the image preset custom field with preset metadata creation in bulk for all existing entries using the Automate.
Tactical Rundown:
The first step is setting up an Automation Trigger to set the trigger for bulk entries, we can choose either a workflow trigger, release trigger, or environment trigger to kick-start Automation for bulk entries.
Here's how the Automation would work:

Let's set up the Workflow trigger in Automate and get the output of entries which is moved to the image preset migration stage.
Note: As a prerequisite, the respective workflow should be configured. Here's the guide on how to do so.

Get Entry data for native asset file field values which need to be transformed to create metadata against it

Use Transform action to Create Metadata Request Body using native file field UID data

Make Create Metadata CMA call against those native field asset UIDs
Transform Request Object for Entry update to migrate metadata to preset field.
{
"entry": {
"banner_image_preset": {
"uid": "{banner_image_uid}",
"_content_type_uid": "sys_assets",
"lookup":"{lookup}",
"metadata": "{metadata}",
"extension_uid": "blt42b2de65c82f3ce3"
},
"asset": "{asset}"
}
}Update Entry with Preset image data in the custom preset app field
While setting up Automate Enable Throttle Execution toggle to throttle automation's executions to help stay within API rate limits while performing bulk update operations on Entries.
Migration of native image field to preset app field completes