Thumbnail size round error – Backward compatibility?

Hi,

I’ve migrated theme and wordpress versionfrom 3.9.3 to 4.1.1 and had the need to regenerate my thumbnails.
since I was using the same size settings as before wasn’t expecting any issues with this.

However, most of the images inserted in posts with “medium size” got broken.
i’ve realized that the filename was, usually, off by one pixel.

and here: https://wordpress.org/support/topic/rounding-error found the reference to this ticket: https://core.trac.wordpress.org/ticket/18532

So, apparently the thumbnail size “rounding error” was fixed in 4.1.

The question is how is backward compatibility guaranteed?
I’ve now migrated a website with thousands of posts and it’s impossible to go through them manually.

It’s ki nd of difficult to understand how this change was done without considering that to fix a small issue a huge backward compatibility problem was created.

Essentially for existing websites, thumbnails can never be regenerated again…

If anybody could tell if there is a way to temporarily put the “error back”, so that I can at least regeneate thumbnails with the old sizes and get to a working version of my website, I would be greatly appreciated.

cheers,
miguel