It would be a substantial reengineering project, not just a different type of cargo.
Besides the weight/balance issues, there is the whole issue of the way the airframe is designed and built. A commercial airliner is a big pressurized tube. It expands and contracts under the cycle, and the movement is consistent throughout the airframe. It's a significant amount of movement that takes place, several inches in diameter depending on type.
So you can't just take the cargo spaces, which are pressurized compartments, and put bomb bay doors in them. You would need to have pressure bulkheads, floors, etc., and separate the crew spaces from the weapons spaces. This changes the characteristics of the airframe and how it handles the expansion/contraction from pressurization and depressurization cycles.
Or you have to operate below 12,000 ft. MSL all the time, or put the crew on oxygen.
To have a convertible role would be tough, since you can't realistically use a non-pressurized aircraft for a transport. Or at least, the areas that are used for weapons would not be easily converted for people.
So it could probably be done, but I think the "Cheap" part would be missing...
