you need your fob code reading from the old becm unit to be able to use your existing remotes,to do this you have to unlock your becm and if the secondhand becm is locked you need to get it unlocked to program all your info on to that.
I have bought a good few becm,s off ebay and alot off them are in becm lockout mode( becm alarmed) the only way to remove the alarm is to have it unlocked with special software.
Your becm has to have all the option,s your car has set on it, ie ,gearbox type,engine type, key code, eka code,sunroof,etc,etc,etc
Regards