Brand new to this kind of stuff, but here's what I'm trying to do and what I tried.
I'm trying to make all the vanilla flight paths cost 0. Not adding any new ones, just making the base ones free.
Opened up the DBC as a CSV, changed the column #3 long values to 0, saved it back out as a dbc, then dropped it over the existing one.
Everything booted up just fine, flight paths still showing normal cost, still attempting to charge me.
I'm using the DBCUtil other people have distributed for this exact use case to convert the files back and forth.
Do I need to make MPQ patches, and client ones, if I'm just modifying and overwriting vanilla stuff on the server's dbc's? I wouldn't think so, but idk.
Is 0 just a number it doesn't like? I remember a flight path from the inside of the dark portal to Thrallmar being costless.
EDIT: Turns out 0 is a number it doesn't like. IIRC, all the free flight paths were only 1 path step / leg. Trying to do multi-leg flights must bother it. Setting everything to "1" works as expected, save that the tooltip still shows the original price.
Happy modding, friends!