`git lfs migrate ` rewrites the commits to convert large files in the repo to/from LFS pointers, so yes it does change the hashes. That's a well-documented effect.

https://github.com/git-lfs/git-lfs/blob/main/docs/man/git-lf...

Now, granted, usually people run migrate to only convert new local commits, so by nature of the ref include/exclude system it will not touch older commits. But in my case I was converting an entire repo into one using LFS. I hoped it would preserve those commits in a base branch that didn't contain large files, but my disappointment was said .gitattributes pollution.