Hi it depends on what stock you have on there at the momement. I've just replaced a tactical stock (like one in picture ) for a folding one. striaght forward job only because of the two screw holes on the sides of mounting. I believe the full stock version has the screw holes top and bottom, this is where you may run into trouble as you can see screw holes on the side . And yes you will have to reroute battery wire. Easy enough to too.Hope this helps
If you have the cm028A (tactical with solid stock) then it will not fit without some modification. The body of the gun will have two tabs at the rear, top and bottom. This stock has screws on the side. There is an article on arnies airsoft about machining your own stock tube adaptor, but I can't find it at the moment. You may be better off selling your 028a and buying a 028c which would have to stock you require and cost less in difference than the £45 the one on ebay would cost. (25+handling fee+vat)
then stick any m4 sliding stock you like on. It may need a bit of modifying to re-route the battery cables to the top cover, or get them into the stock tube, then you will need another battery as you will either have to have a stick for the top, or a nunchuck type for the m4 stock.