I am often asked to help people forward their ports for certain online games. I'm successful almost always, except when they have an IP address starting 10.xxx. If it starts 192 then its fine and I have 2 questions. Why do I run into problems with the class A IP address? Is is possible to change the class of the IP from A to C (10 to 192) for these select few modems and routers?

You're problem might be the subnet mask. A Class A subnet mask it 255.0.0.0 Class C is 255.255.255.0 What error messages are you getting?

All the rules are the same for a class c as for a class a. I would suspect the class of ip has nothing to do with your issue but its one of equipment. And that would be its ability or inability to forward. After all there are no port differences in a class a vs a class c address space. You can put a class a subnet on a class c address which will result in 16777214 possible subnets. You can not change publically assigned ip. You can only change private ip which is on the lan side of the router. Example of Oxymoron: Person who is pro life and anti sex education. Education is key to prevention. Prevent conception you prevent abortion. Abstinence training clearly isn't working.