Deductions is simple,
At 12 AM dbr amount will deduct as per you pack price for upcoming 24hr.
if you modify pack and your new monthly price increases then airtel will deduct differences of new dbr, but your new monthly price decreases then no refund for deducted dbr amount, and if pack price is same after modification then no extra deduction.
For example
1- Your monthly pack price is Rs-300, So your DBR is Rs-10
At 12 AM airtel will deduct Rs-10, then on same day if you add channel/pack/bouquet priced Rs-15/m, and your new monthly price is Rs-315 and DBR is Rs-10.5,
Then airtel will deduct Rs-0.5 instantly at time of activation.
2- But if you remove channel/pack/bouquet priced Rs-15/m, and your new monthly price is Rs-285 and DBR is Rs-9.5,
Then airtel will never refund already deducted Rs-0.5
3- But if you remove one channel and add another channel priced same then no deductions of any money, but try this (add&remove) in one/same attempt or first remove any channel/pack/bouquet then add another of same price.
So in pack modification you can Remove Zee TV HD and add StarPlus HD, then no extra amount will be deducted.