Well, laying new paint on top of old paints can sometimes cause issues. We don't know what kind of paint is used on that frame. You may end up with bleed-through or snake-skin effects (from laying down fast-drying paint on top of slow-drying paint).

The other thing is how nice of a finish do you want? Sanding down tubing is difficult and paint is a lot softer than aluminium. So if you just sand the surface of the existing paint, it's very easy to end up with waves and uneven finish because it's hard to sand tubing perfectly even. It won't be obvious because of the rough surface, but will be very evident when you lay down a fresh glossy finish.

If you don't want to strip the bike to bare metal, use sandpaper tape to wrap 360-degrees all around the tube at an angle and "floss" it back and forth to sand the tubing perfectly even all the way around.