No, it doesn’t seem that he will change those laws. Those laws are mentioned in the Qur’an and so he won’t change them. However, the Imam will stop illegal forms of enslavement. Peace will spread around the world. Wars will end. The evil ones will be uprooted. Hence, there won’t be enslavement after his government settles and people submit to the path of goodness.
We know 100% that Allah is just and merciful, and all His laws are based on wisdom. We may not understand how His system exactly works, but we know it’s a just system.
If there will be slaves in the era of Imam Mahdi (a), then we know:
1- They will be justly enslaved, not unjustly
2- They will be treated well, based on justice. Their rights will be guaranteed
3- The Imam will encourage their owners to free them