It's not a "window support". Are those two J strips on top of that siding?
Looks to me like the guy who did it did not know how to install siding. He should have put his starter strip higher so he could finish at the top properly.
If I am seeing it correctly. I would tear out the wood that's directly under the windows and create a sill out of PVC 1X4 or 1X6 that will provide ~ 1-1/2 inch overhang or projection.
Then remove the two "J" strips (if that's what they are), then remove the siding from the top down.
Raise the staring strip so you can trim the top piece of siding (cut just below the locking piece and add an undersill trim piece
to finish the top properly under the sill.
The amount you cut off the top piece is the approximate amount you have raise the starter strip.
You don't have to remove the vertical trim pieces.
The miter angle on the sill is probably 22-1/2 degrees, test it with scrap first.
Not as bad as it sounds.