Replacing existing electrical line to outbuilding
You need to tell us what size/type of conduit you have, what size/type wire or cable is coming out, and what size/type wire or cable is going in. Even why you are replacing the wire/cable might give us some hints, if some of the info is difficult for you to obtain.
For all we know, you may be attempting the equivalent of "pushing a new 3/4 steel rod into an old 1/2 inch steel hole".
The pull rope comment was made while rushed. Should have been prefaced with if you attempt reusing the conduit. If the task is possible, it will make it easier. If something goes wrong, it is cheaper to cut and abandon rope than new wire.
If you give us the info that has been previously asked for, you will get better answers.
Without additional info, the best advice I can give is: start digging a new trench.