"why it cud be even like rahman already wud have composed some tunes for a producer which gautam is now keen on using, or in the worst case, rahman's music for a hindi movie being re-used"
Composing tunes is no big deal for ARR -- He once said he can compose six tunes in half an hour. He usually takes time to finish the final product. I am not sure Goutham will agree to re-use ARR's hindi songs either. I guess he probably got sick of Harris's repetitive music and wants a change. In any case, he is in for a long wait and the December release may not materialize if ARR is the composer.