If taking public transport, wouldn't it be simpler to take the RER B to Chatelet-Les Halles and then transfer to Line 1 at Chatelet? Or is the station layout such that it makes sense to transfer to RER A first?
If travelling in one general direction, RER B to RER A is a simple cross-platform change at Chatelet, but, as the OP needs to go in the other direction, he can't do this so would have to change platforms via the stairs or escalators. Still very easy to do - transfer to Line 1 involves a very long walk, which may be a little easier to do at Charles de Gualle Etoile.