No. Warheads come as a whole package, along with heat shield. What you are saying is the canopy and warhead bus or Post boost vehicle which can be changed for different types of warheads and canopy keeps the missile aerodynamic shape intact while going up. On the way down there is no canopy and warhead's own heat shield has to work.,and it cannot be retrofitted on the spot. But yes, the actual "Atom Bomb is a separate structure residing inside the heat shield.
We saw this clearly in North Korean Hydrogen bomb press released? Kim is stood beside the actual nuke and the Heat sheild is the green thing in the foreground,also explained in the diagram hanging on the wall.
View attachment 1477
Missiles are not general purpose vehicles as you put it. SLV are general purpose. Any misisle is bespoke made to carry a certain type of warhead.
No wonder India made missiles to carry nukes and didn't use SLV . Both are rockets and go to space,don't they?
Also look around. With every Missile you will see designated type of warhead. Not that it will carry any available warhead and the rest of the space in the canopy can be filled with Styrofoam as you put it.
For example W76 assigned for Trident 1
W88 and W76 assigned for Trident II
W78 , W87 for Minuteman-3
Likewise India or any country builds missiles for certain warhead not as general purpose missile.
However i agree that missiles can carry certain designated types of warheads , but in that case there is visible evidence.
EXample isMinuteman missile in different visibly differentiable configurations for carrying different numbers and types of warheads. (This is a Museum Display picture but depicts actual configuration)
View attachment 1478
Have we seen any Indian Ballistic Missile in such visibly different configurations? (Except Shuriya K-15)
You are free to post any pics in this regard.