[Quick-Question] Downgrading A Patch

How would I downgrade a higher patch of conquer to a lower patch? is it the encryption keys, or something else?

for example, downgrading impulse's 5165 to a lower patch. i.e. 5095 or even lower to 5017 etc.

sorry for the question, but I was very curious for a while now on how to do this.

any help will be highly appreciated.

Packet structures, game sequences, encryption systems and encryption keys.

Mostly just packet structures though. 5018 is full encryption change but both systems and very public so just use whatever you want

My first "custom" source ever was influenced a lot by Impulse's source. It runs on 5017. You can use it as a reference. I lost the database for it.

It's actually relatively easy after you have found out what to do (I semi-downgraded Project Manifest and Albetros to 5017).
I'd start with the authentication progress (remove the new password encryption if needed).
Then removing blowfish would be a good idea and adding the necessary parts to make the packet encryption work (5017 uses the same encryption for auth and game but you need to switch the keys used in the encryption progress to key 3/4 from 1/2 to get game encryption working).
Then downgrade all the packets which should be easy since most structures are public.

//edit Really important, don't forget to update item database serverside, failing to do it may cause you weeks of sleepless nights because of weird problems occurring due to having not working armor IDs for example, I speak from experience.