Why its not practical ? For sure India must be have diffrent playload for different mission . It is practicle . Its not just theoritical . Modifications will be made as per the requirements .
Its simple as that
When a missile is designed, the range and payload (and velocity/acceleration/g forces etc) parameters are specifically set to define everything that will be used to develop it. Considerable re-designing efforts are required to make modifications like you mention.
There are a lot of factors which need to be considered:
1. Different payloads= Different warhead packages per missile. How do you propose SFC will deal with this level of complexity, besides that already in form of range envelopes? It can't be like
Major: Sir ji kitne wala chahiye? 1 ton wala le aon ya 1/2 ton wala kaafi rahay ga?
Remember, Strategic Forces require simplicity at each level, so that when the time comes, there are not many decisions which need to be taken. Just take a look at US/Russia/China's arsenals. Nothing like variable payloads exist anywhere. For multiple strike ranges, entirely new systems were developed.
2. This will also result in increased number of missiles per type, which is unnecessary, costly and not feasible.
3. Also, Lighter payload= More speed/acceleration= More physical stresses. How can you guarantee that the original missile's body and components can take the increased amount of forces?
4. Furthermore, how would you make sure that the ReV will re enter smoothly despite reduced weight but same size? The density change could effect the re entry, to that point that the ReV could wobble if it isn't stable.
Besides, a fixed amount of weight in the Re-entry Vehicle (the total payload) is that of:
1. Nose cone with heat shield/ablative coating
2. ReV engine+fuel/thrusters for course correction
3. Thrusters for spinning warhead
4. Arming and Safety electronics
5. Detonation mechanism
6. Self-destruct mechanism
These all, combined, have a total mass variable from 1/5 to 1/2 the mass of total payload (depending upon weight of the nuclear device). So basically you don't really have much room for reduction of payload.