Ques. Is hard reset of I phone changes the version from new to old?
Ans. Well no, downgrading an iPhone is nearly impossible. Once you upgrade an iOS device there is almost no way to go back. The only way would be jailbreaking the device and then flashing a previous version of the OS on the device, but this is a very risky, cumbersome process and you will lose your warranty. And if not done right you might brick the device. So if after upgrade, your device feels slow we recommend you to wait for some time as Apple is in the process of preparing new updates for the IOS 8.1 version which will fix these performance issues.