Replace (overwrite) Previously Uploaded Images (Including Thumbnails!)

Hi,
I have a ton of images already uploaded to a wordpress site. The problem is that I need to overwrite most of these images with new version of theses images that are :
1. Better optimized (smaller filesize and lower resolution in some cases)
2. Enhanced to correct white balance issues, sharpness, contrast, better cropping etc… note that some image might need to have their proportion changed.

This should have been done before, I know, its too late now and I would like to avoid re-uploading the images manually through wordpress one by one and going to edit the image urls links in each post, this would be a real pain…

I would like to edit the images in batch with lightroom, then simply upload them through ftp and overwrite the old ones, but my understanding is that this will not work well because wordpress automatically generated thumbnail of various sizes when I uploaded the images for the first time, so anytime my theme uses thumbnail I would be seeing the old versions…?

BTW I dont know if this has any impact but i’m using a premium(paid) theme