1) I guess you have three alternatives: 1. Find a Q_NMOS symbol with different D/G/S order. 2. Modify the symbol. 3. Modify the footprint.
In KiCad symbol and footprint are independent, you can attach any footprint to a symbol. The footprint only has to have at least as many pad numbers as the symbol has pins, and the application attaches a pad with a certain number to the pin with the same number. So if you modify the symbol or the footprint you have to swap some pin or pad numbers.
2) Yes you can, it's a basic operation, it's difficult to imagine how one could design a complex project without doing that. But if the new footprint has a different layout or pad order than the previous one you naturally have to re-route something.