Does the siding but up to the window frame or is there a trim board? Is the siding beveled, T-111, what? There are 3 ways you can go if the siding buts up to the window.
1. Remove the slider/ remove the glass, colapse the aluminum frame, install retrofit windows without the fin.
2. Take a circular saw, set the depth at 5/8"(+/-), Use the window frame as a guide for the saws base plate, and run the saw around the window. Remove the 1 1/2" scrap of siding, pull the nails, and remove the window. Install New construction window, add 1 1/2" trim around the window and run flashing on the top of the trim/under siding. Caulk & touch up paint.
3. Remove siding, Remove window. Re install etc, etc.......
Make sure that the depth of the fin to the inside of the new window is the same or slightly less than the old ones.