Does Cancelling a Sitting effect me negatively?

When you use the "Extend the Stay" feature, it will create a new booking for the added dates. This ensures that both parties have the chance to look over the new dates and approve. When using that feature, you shouldn't have to cancel the original booking. Just choose the dates you want to add, and the extended booking will only cover those new dates.

For example, if you click "Extend the Stay", you'll be able to select the dates. If the client is already booked from 9/25-9/27, but needs an extra night, you'll only select 9/27-9/28. This way, the original booking covers the first portion and the extension only covers the additional night needed. No cancellation needed.

When all else fails, you can contact Rover Support to make edits on bookings. But as Karen mentioned, as long as you are not actually canceling on a client, you will be fine.

From what you've described, you have not been cancelling any stays. That can only occur after the client has paid and you've backed out on them. You are merely fixing a booking request before the client pays.

Cancellations do affect not only your position in search results but can get you suspended by Rover. However, it does not sound like you are actually cancelling anything.