Anytime before your contract closes, you can ask to have it rewritten with the new incentives.
I added on in March and was able to change my deal when better incentives came out April 1st.
When I added on again in June, I planned it a week before the incentive change. They were worse for BLT so I kept what I had.
At the time, it took about a month from signing to close, but I have heard it is taking less time than that now. Once the contract closes, no changes can be made.
Good luck!