First, sometimes they try to get a new patent that will cover some aspect of the old process, too.
Secondly, a lot of effort goes into getting people on the newer formulation. Look at Concerta versus Ritalin. Both are methylphenidate, but I've met several people that think Concerta is a totally new drug and that "Ritalin didn't work for me". I've even talked to doctors that thought they were different chemicals. Not the drug company's fault, but it still "hurts someone".
Hurts? Immediate release methylphenidate has a half-life of 2-4 hours, while Concerta has a duration of effect of 12 hours. Given the narrow therapeutic window of methylphenidate and the harsh wthdrawal syndrome, they are different drugs.
Secondly, a lot of effort goes into getting people on the newer formulation. Look at Concerta versus Ritalin. Both are methylphenidate, but I've met several people that think Concerta is a totally new drug and that "Ritalin didn't work for me". I've even talked to doctors that thought they were different chemicals. Not the drug company's fault, but it still "hurts someone".