The Simpsons hasn't changed a whole lot over the course of it's run. I mean this in a positive light of course.
True enough, it doesn't make me laugh as much as it did so many years ago, but keep in mind that humour is a personal thing that changes as one gets older.
I began watching The Simpsons as a child, and I laughed long and hard at most of the jokes. Flash forward 15 years however, and I laugh at a quarter of what I used to laugh at as a child. The show still amuses me, keeps me entertained, but it's just not 'knee-slapping' funny anymore; mainly due to my age and what I like to think of as a higher level of maturity. ;)
The Simpsons is a classic. It's not meant to have a major overhaul every few years to keep it entertaining. In fact, if it were to change so drastically, it would most likely lose a good portion of it's fan base. A fan base which is primarily made up of 10-15 year olds I might add; who probably find it as funny as I found it at that age.
It's a good show, maybe not for everyone, but for those of us that like it... it's fine just the way it is. (Anything tends to get a little stale when it's around for 20 odd years).