what do i have to do to force the client to start in classic mode without copypasting an renaming files?

inside server.propreties file from config folder , search for the line where you need to select your protocol version, and chose the 19 or 28 then save, open server and login with the classic client using the protocol 19 or 28 system 

but the oddyssey client is a hybrid that consists of odyssey and classic. how do i change the client so that by launching the l2.exe the client starts in classic mode and not in odyssey mode?

 

otherwise i have to delete all new files and rename the classic ones - for example "weapongrp_classic.dat" into "weapongrp.dat"

It might be command line argument, take a look at them.

I remember that L2JMobius had an option to activate Classic Mode. I think is all related to the packets that the server request depending of the protocol.

i told you already, you need a classic system