Hi,
Please let me explain first my though about image and SEO:
Unlike a lot of people I don’t think image path would change anything to your SEO, it may have been one of the suggested optimization back in 2012, but nowadays nothing tend to prove that it may work anymore.
I do think that image name, alt text, structured data, image quality and optimization is far more important than the image path.
Note that I make a clear distinction between image name and path.
By the way nowhere in Google documentation image path is mentioned
https://support.google.com/webmasters/answer/114016?hl=en
I would more change uploads/apple/iphone7/1.jpg to 2018/11/iphone7-front.jpg that would fit more the Google recommandations.
From my point of view it doesn’t worth it (please refers to my technical informations at the end of the answer)
Now let me explain exactly what my plugin does.
It allow you to change the attachments path and name.
It has also the integration with WP Media Folder from Joomunited, when a file is moved into any Joomunited’s virtual folder, my plugin will move actual files in the server to reflect the virtual path.
If you have already added all your images to the library and if they are sorted in the folders you want, then you just have to install the plugin, go to the plugin settings page and click on the “Move existing media” button.
It will move all your media which are in the upload/… folders to actual folders based on the virtual ones created by WP Media Folder (from Joomunited)
BUT as you have 50K images, I also assume that you have a lot of pages and posts, it will need a lot of server resources.
As WordPress is not intended to allow moving files like my plugin does, the process is a bit complex.
It will search into every column in every table in your database to replace the old image path by the new one for each image you use.
It means 50k images * 20 tables (for a small website) * 10 (columns) = 10000000 databases update requests.
It requires a lot of CPU and memory.
Also if you use plugins that deals with attachments on a way which is not expected this could lead to broken images links in your database.
You must make a full backup of your website before trying to move all your files.
As I said previously, I don’t think the SEO benefit (if there is one) worth it.
I would suggest to focus on other optimizations that may have a better impact with less possible problems.
Hope my explanations are clear enough.
Best regards