CSO, WSO, EWO to Pilot all possible. But those who decide to switch early in their career usually make the switch within 1-2 years after FRS and while in their first tour while being a back sitter. How hard? It depends on the demand and the needs at the time. Not many apply to switch to Pilot, however. There’s few reasons why. One, if you want to pursue a long term career in the Navy or the Air Force, making the switch can hold you back with the rest of your class to be competitive for the O-6 and even to Flag Officer Appointment. Most who switch end up retiring after O-5 because of this.
Good thing is that there is a big need for more pilots in the Navy and the Air Force. So the chance is good. But, in the Navy, I am told about 3 switch to Pilot on average each year. And the priority is given to those who have strong FITREP and would plan to come back to the same squadron. So if you’re a CSO on F15 and want to fly the F22 or F35 at a different squadron, perhaps, that can be more challenging. You can reapply if you don’t get it after first application.
You can also leave the Air Force as a CSO and join ANG. This way you can go to UPT with a choice of aircraft you want to fly with a guarantee. As a CSO, you are already trained in everything except the stick time. To be most competitive, both the Air Force and the Navy like to see that you already have PPL which shows basic airmanship and motivation. You can also go straight to Primary to fly the T6 (Navy) or the T7 (Air Force).
I believe you can switch up to the age 35 at the time you start UPT. With a special age waiver even up to 38. Those who switch after WSO, EWO, CSO, do so at age 30. Your age depends on when you got your wings. But I wouldn’t wait too long to switch if you plan to make the switch.
The process is complicated so if you want to learn more about this, you should sign up for BogiDope.com. A lot of info there on this topic.