Find a matching router bit profile and duplicate the profile and return. Cut the existing return off with a Japanese pull saw. Fit the new longer profiled edge with return and fasten it in place. Remove the lower molding, cut a matching longer piece, return the end, and nail it in place.

Good thing it's all getting painted.

You will need a pro for that one. The horizontal sill needs to be extended. As Joseph suggested above, part of it needs to be cut off and then a new piece added. It isn't a DIY job. It's likely not the news you wanted to hear.