The grade history log would reflect both grade changes as separate entries, clearly indicating the before and after grades.
The grade history log only reflects the most recent grade change, and the earlier grade changes appear to be lost. It appears that the only way to log multiple grade changes is to change them on different days (which is definitely not a great workflow.
I saw this in the new beta release notes.
From the screenshot it looks like it will now capture multiple entries on the same day. I am assuming that this gradebook history update will apply to the new gradebook as well.
As Todd indicated, our previous release introduced new Gradebook History functionality. I haven't been able to not have multiple grade changes display in the page, so you should be good to go!