Replacing windows sills

The window cill is attached to the bottom of the window before installation it's unlikely you can fit it property after. It has a lip on the back to prevent water ingress and the front is normally wider than the window . You may be able to cap the tiled cill with fasia board ? Hope this helps.

Although you probably could it’s so easy to remove the window there would be no point not taking it out and it would make it far easier to change the cill and would be a far better job with the weight of the window sitting on the cill correctly.

This would depend on wether you’re referring to the interior or exterior cill , if interior it would be possible to do without removing window if you’re talking about exterior then I would presume the window is face drain so would need removing and placing on a cill to avoid drainage getting blocked hope this helps