Window trim-Do I need to remove existing trim?

I think this could be done but you will need to rent a table saw and do some staining of trim pieces. I will share with you a photo of the trim we just recently had done in our sun room with hopes it will give you some ideas.

Yes

yes old trim needs to be removed to replace with wider trim.

No problem. I changed mine and didn't use a table saw just a mitre saw. Remove what is there and measure the lengths on sides, top and bottom of the window then cut your new pieces of trim and mitre the corners. Install with finishing nails, fill and sand holes, paint or stain. Voila new clean trim.

You will need to remove the existing sills, facings, other trims and the windows. Be sure to caulk the entire rims and every seam related to the windows. ☺️