This is where I always put a few caveats to dgoldenz answer. Technically this is a matter of state law and if your state law does not bar re-underwriting, the insurance companies can raise rates based on claims. Many states do bar this practice. I don't know about Missouri. It is very unclear how common this practice is in states that do not bar it but it certainly has happened. It may be the case that it occurs less or not at all with "name brand" insurers.
What is more common is that the rates for the entire group may go up, especially if the people in the group are aging or have a lot of claims. The healthy ones leave the group for better rates and the rates go higher and higher for the ones who are left and can't get underwritten insurance.
Or, the plan is terminated and the healthy people are able to get on a new plan and the unhealthy are not.
So, just because you get a plan and are healthy now is no guaranty that things will not change in the future. I wonder how many people on individual plans end up on the same plan for decades. Not many I would guess.
A few years ago, people would keep the same plan for 5-10 years because rates were stable and deductibles were low. Now the average is more like 1-2 years because of rate fluctuation, plan designs, and HCR. I would venture to guess the average HIPAA case stays on the same plan 3-7 years. Of course, this may all change in 2014 if HCR isn't repealed or changed.