L2J Server

Well skipping the blah blah, i want to downgrade interlude to c4, lets say i pick acis as my base since its the better IL option, i would pick current h5 but the distance between h5 and c4 is huge and because on another topic i saw that downgrading is mostly deleting stuff i thought IL is way closer.

Anyway

What i want to ask is where do i start from?
My first thought is changing all the packets to the c4 ones. Will that at least allow me to enter using C4 client?

What other steps should i take?

It's not that simple.

You should change the protocol version and try to login with the C4 client and debug the server and check what the client sends.

You must also check the packets IDs.

After you pass the login server, write again here

If you base on something other than L2j, I will not provide support, and you should go to their forums

(If you try to hide from me that you use something other than L2j, I'll figure it out and you will lose my respect and help.)

There is a certain issue with protocol versions 656-709 which ensures that you cannot use the officially distributed client 'as is', therefore this discussion would definitely descend into "client modding". Still, I will take a leap of faith and assume you want to use the korean 654 client, to which this issue does not apply.
(sorry for being so vague, these sorts of things are banished from this forum for a reason)

C4 uses an older auth method than IL (actual change was mid-C5, I still remember that good enough), but the only change is in AQ_LOGIN credential block offsets. (Of course, you can just infer offsets based on size field values inside the credential block and thus handle both C4 and later)

If you need annotated packet structures, I have already finished packet structures for 336-660 protocols for NetPro. Beware incremental packet loading, so if a packet structure hasn't changed in C4, you will need to visit previous folders.