If you're OK with SQL, just move the images to a the new storage, or, if you are looking to consolidate, merge the images on the external storage with the images in the current storage, ensuring you keep the file structure the same and then run an SQL query in whatever panel you use to change the value of the storage to the one you are going to use for those images.
So if you're moving everything from an FTP storage that has a storage ID of [whatever], update the chv_images table to reflect your change, so something like ....
UPDATE chv_images SET image_storage_id= [where they are going] WHERE image_storage_id= [where they are now]
If just moving them back to the origianl server, if you've decided not to use an external anymore, the value of the current new storage will be NULL
UPDATE chv_images SET image_storage_id= NULL WHERE image_storage_id= [where they are now]
Remembering to backup your db first, just in case.
I trialed it from a swift stack back to the hosting server and consolidated two external FTP's using this method, to test the theory.
@Rodolfo may shout a reason why this isn't a good way to do it (hopefully, then I'll learn a better way to do it as well), however, it worked successfully for my trials. If Rodolfo can't see any theoretical problem with that I'm going to write a PHP script to automate the move so I can swap and move storage as I want/need to.