Actually sir,i waited for your PNR number for too long.No response came till then.And there was thanks message after seeing which i thought you have got the solution and this matter is resolved.
After then i got busy in my office meetings and i couldnot see the same.
After a full hectic day again i am present at IRI.Sorry for being late in reply.(In fact i can't help myself even in this regard.as work is work).
Now...
more... coming to your point.
For boarding point change you just visit the PRS with ticket+ID card+application.
example for application:
In application you just mention that because of some unavoidable circumstances you wont be able to board the train at Allahabad instead board the train at Kanpur Central.You will not claim the refund for the boarding point change.(Plz mention this also,as railway would not pay you the difference for boarding point change.
This is just for example.You plz. site your own reason and draft your application.Hope this helps you.
Move to PRS at the earliest.Because even in worst case if boarding change is not allowed in your case(your case is bit complexed) you will have sufficient time for the other solutions if any.But as such, hope that your boarding point will be changed by the PRS people.