Hello @Kathleen21,
As far as I understand it the conditions in place at the time the guest made the reservation request are the ones that apply if you accept. This would mean that your old security deposit value would apply.
Your new value will apply to all booking requests made after you changed it.
Airbnb's reasoning is that it's a confidence and security measure for the guest. It's to prevent the odd unscrupulous host having a policy to, say, advertise a very attractive low price and then demand a higher payment from a guest who is interested enough that they send a booking request.