Change "Full URL path to files" for upload Images?

My site is a car wallpaper provider. I have lots of photos in my site GALLERY. I’m using dedicated sever.

Currently my server load still normal, but no longer able to accommodate the addition of more photos.

I do not want to use CDN (Amazon). I already rent others server just for images hosting my site.

I follow this tutorial: https://hellboundbloggers.com/2010/06/13/host-images-of-wordpress-blog-in-subdomain/

I create new sub domain for images >> img.domain.com

When i change my “Full URL path to files” to “img.domain.com”, all photos on the site Error, does not appear.

But when i try to upload the new photo, it worked fine. My new photo change to sub domain >> img.domain.com/2013/02/file.jpg

Now I know why my old photos Error, because all URL photo is replaced into sub domains.
from: domain.com/img/2013/02/file.jpg
to: img.domain.com/2013/02/file.jpg

My question is: Can i lock yang old url still using domain name and new upload photo using sub domain??