Security Forces to Combat Controller?

I am joining the Air Force in January and am going for Security Forces. I have been looking at becoming a Combat Controller, which you can try out for right from enlistment now, but the failure rate is extremely high. I read once that if you choose combat controller and wash out you can pick any job BUT security forces (not sure if its true or not). So I'm thinking why try if I know at this time point in time, I will not make it through. So my main question is .. Is it difficult to change over from Security Forces to CCT? If not, when in my term of 4 years am I able to make the transition? And if, god forbid, I wash out, am I able to rejoin Security Forces or will I have to pick another job? It seems highly unlikely but I have to ask.