Update fields to use a different FieldFormatter during an update hook
The plugin definitions you've posted are for new fields to build the options when configuring a field formatter in UI. For existing fields you have to update the field formatter in config.

For example:

use Drupal\Core\Entity\Entity\EntityViewDisplay;

function mymodule_update_8001() {
  foreach (EntityViewDisplay::loadMultiple() as $display) {
    foreach ($display-&gt;getComponents() as $name =&gt; $options) {
      if (isset($options['type']) &amp;&amp; $options['type'] == 'old_formatter') {
        $options['type'] = 'new_formatter';
        $display-&gt;setComponent($name, $options)
          -&gt;save();
      }
    }
  }
}