The classic 'trousers to pencil skirt' remake. I've made quite a few of these over the years. Wide leg trousers work best. Unpick (rather than cut) the inside leg seams to give you a bit more material to work with. The weak point is definitely the slit at the back. Make sure you make it long enough otherwise walking can be somewhat restricted. Of course you could put the slit somewhere else such at the sides or front centre or just make the whole skirt shorter.