Cancellation and Booking of MCS/ Adv ITT batch

Since the online registration portal of icai won't let you book another batch for same timeline, it only allows you to book another batch for same timeline only after cancellation of the previously booked batch.

I want to know if I cancel a confirmed seat of MCS/ ITT, how much time does the portal takes to update my cancellation and allow me to book another batch for the same timeline.

My fear is, if I try to get another batch with less number of seats (2 or 3) by the time my cancellation is reflected on Portal and it allows me to book another batch, the seats may get filled and I would have lost my confirmed seat.