I want ton understand your excatly requirement. What i got that you want to update 2500 posts’s field named as “theme2035_embed.” and need to put value of field named as “video”.so is a fixed value of field named as “video” for all 25000 records?
you can update the admin bar if you get success response from ajax request and also return dynamic admin bar with user logged-in details in response and replace this response with existing admin bar(Add some id this this existing bar to replace html using jquery)