rotten wood sill

The wood sills on some of are windows are rotten. Should they be replaced before we replace the windows or will the the new windows sill and new outside casing be sufficent?

Depending on the degree of rot there are some easy remedies. If the rot is only on the nose of the sill it can be cut off and a new piece attached. If the rot is high inside the sill board you should think of replacement. If it's just flaking off some paint and still has plenty of strength and durability just cap over it. If you can punch your finger through it or break it apart by pulling at it. Get new ones then wrap them. Good luck!!!