Can't edit mosaico templates any more
Mosaico extension stores path in civicrm_mosaico_template and civicrm_mosaico_msg_template table. When mosaico extension code is moved you will need to update path in these tables to new path.
You can run below query to update the path from sites/default/files/civicrm/ext/ to wp-content/civicrm_extensions/
UPDATE civicrm_mosaico_template 
SET metadata = REPLACE(metadata, 'sites/default/files/civicrm/ext/', 'wp-content/civicrm_extensions/‘);

UPDATE civicrm_mosaico_msg_template 
SET metadata = REPLACE(metadata, 'sites/default/files/civicrm/ext/', 'wp-content/civicrm_extensions/‘);

HTH
Pradeep