Ticket Cancel?!
 

My family and I have bought a ticket to India. We were planning to go to Chennai India via Delhi, but a family emergency came up and now we plan to go to Bangalore, India by jet airways which is code shared with American Airlines.
For some reason now American Airlines said if we plan on going to Bangalore we will have to cancel phase 1 of our return ticket to Brussels, Belgium.
But our Phase 2 return ticket from Brussels to Chicago is still good.
I don't get it.
I need some advice people.

What was the exact original itinerary? (mainly need to know what cities, and what airlines, and if any are codeshare tickets or not)