Wood Rot
In most cases we can change the existing window style to a different style.
The biggest issue will be the height and width of the window, because each window style has a limitation on the size that it can be. There are pros and cons to each window style and the decision generally comes down to personal preference of looks and how you would like to ventilate.  One nice feature of Double Hung windows is that it is easier to ventilate when you have limited space on the outside to open the window.  You also get better ventilation since it will allow air in no mater the direction of the wind.