You can replace the 'original' with your model. But, usually you shouldn't do this because it may affect other addons that maybe reliant on that specific model etc. etc. You should do it the way Gnat mentions and create your own unique class variant name that inherits most of the properties/attributes of the 'original' BIS variant and then just overrides the portions you need to override.
If you've taken this into account and still want to completely override the previous class implementation then doing something like the following would override the model....
Code:
class CfgPatches {
class SyWeapons {
units[]={};
weapons[]={};
requiredVersion=1.02;
requiredAddons[]={"CAWeapons_E"};
};
};
class CfgWeapons {
class M249;
class M249_EP1 : M249 {
scope=2;
model="\sy\weapons\sy_m249";
picture="\ca\weapons_e\data\icons\m249_iron_ca.paa";
displayName="Sy M249";
irDistance=100;
magazines[]={"100Rnd_556x45_M249","200Rnd_556x45_M249","30Rnd_556x45_Stanag","20Rnd_556x45_Stanag","30Rnd_556x45_StanagSD","100Rnd_556x45_BetaCMag","30Rnd_556x45_G36","30Rnd_556x45_G36SD"};
};
};