At most airports, I suspect that you'd need to get the help of an airline or airport worker to let you through a secure door to get from the departure hall to incoming immigration.

The one exception to this that I've experienced is the old international terminal at SEL. At that airport, all inbound passengers and their carry-ons had to pass security screening immediately upon arrival, before leaving the gate area, even if they were not connecting. Once past this, you entered a combined arrivals / departure hall. All international passengers were co-mingled in this hall: inbound, outbound, and connecting. So, if an outbound passenger were to change their mind, it would simply be a matter of getting in the inbound immigration line with all of the arriving passengers, although some explanation would be due at immigration control.