Gift Deed Cancellation / Update

i) A and B own a flat in mumbai, A prepares Gift Deed for transferring the flat to B, after one week they decide the previous 50% ownership was fine and want to cancel the gift deed with mutual consent 1) What is the process 2) Is there STAMP duty to be paid for Gift Deed Cancellation 3) Any other fees ii) Can the Gift deed be modified so that A to make it conditional gift deed where A should be the 50% owner till A is alive