You can extract the Activity Log via API and you'll see the Change Score activities.
Note Change Score is going to start aging out completely after 90 days in the new activities infrastructure rolling out later this year. In the original infrastructure (still in place in June 2018) activities would age into an archive area, where targeted queries could still read them. Under the new rules, when they're gone, they're gone.
Post data retention policy update - as per best practice, you should create datetime fields to save timestamps when a lead reaches a stage. You could create a field to record their current score in the same way - though most people set MQL status when a lead's score reaches X. Curious to learn how come your set up is the opposite?