I rented a house with 2 roommates and decided to leave because one of them was very noisy at night, not clean and not respectful. This roommate was also the landlord. I found someone to rent my room with my roommates agreement so I left a month ago. The landlord finally agreed to give me back my security deposit but after I left the house he decided to change the door locks because 6 months ago I lost my key so he deduced the money from my security deposit. I was wondering if he's allowed to do that without asking me first, especially because 6 months ago we agreed not to change the lock and that I can just duplicate his key.
Thanks

Yes, he can do that. A landlord is not required to ask a departing tenant for permission to make repairs before the LL can do the repairs and charge them to the security deposit.

If he then gives you a statement of charges against the deposit in a timely manner, you would have course be entitled to claim that either

it wasn't damage but normal wear and tear;
you didn't cause the damage; or
the cost of repairing the damage was excessive.

In this case, none of those applies.

While I think it is sort of unfair and not a nice thing to do, since it was discussed before, but yes he can.