Hi Ed,
There are a lot of details that you have to look at before you start on an S5 to S7 upgrade. Like Anton said, you have to look at communications, addressing in the S5 PLC, modules like IP's (intelligent processors), CP's that are not supported in the S7 that use older protocols, etc,etc.
If your PLC only has I/O in it your upgrade should be fairly easy. You should know your process inside out before you start. Siemens has a conversion program but it does not convert everything, ie hex addressing.
Are you just upgrading because the S5 PLC's are obsolete? If so there are other options where you might be able to still use the program you have and simply change the hardware. There are two options with Vipa. You can change the hardware and use the same PLC code or you can rewrite the code in step7 and keep the same hardware and simply change the cpu and add a profibus card to your s5 rack. The latter requires that you have a 115 or 135 rack with I/O modules only. check em out. They have some pretty cool stuff.