Simply put, don't. Birdman is right... to an extent. There are times that carriers will no longer support a phone or a system, but that doesnt happen often, more times than not, it is because people are under the impression that if thier contract expires they will not get cell phone service, this is a myth. Others are just persuaded not by misinformation per say, but by creative advertising.
Truthj be known the best system ever out was the analog system, however because of strain and expenses it led to lower battery life, and higher costs.
But like the expression goes they dont build them like they used to. For example the Nokia 5100 series phone was probably the best series on the market and arguably still on the market although it has evolved so much it looks totaly different.
If you have a handset that you like, and it works for you hold it as long as you can. But if you ever think you want to change I would recommend staying with a nokia. They have a very old world style of thinking, which is if its not broken dont fix it. They are fairly easy to use, sturdy phones, and well tested.