Sachin,
Interesting question! Thanks!
Switching modules is not same as a fresher trying to enter in to SAP market. But many of the same challenges will be there is switching too!
In this case, you know SAP but you are no more an expert in the new module you choose. That will bring with it difficulty to find a job, low salary, effort, hard word you have to put will be much higher - as you will be in a learning phase.
If you have a love for that domain and domain experience, then go for the new module. Or else it don't make much sense!
TW