I got 650cc. Can I downgrade to a more natural look? Maybe 500cc 450cc (Photo)

Your youthful (biologically) skin will redrape and shring around the downsized implant, without any other maneuver rather than the compulsory capsulectomy and, well likely, a multipoint capsulorrhaphy to relocate at higher position the overly low current submammary fold and, probably, also set the cleavage a bit less medial (now you are close to develop symmastia).
Downsizing massive implants is a technically challenging and critical procedure, not a simple one as many surgeons and most patients may believe; it is not just an in-out gesture, a series of new pocket redefinition gestures have to be applied in order to avoid a grotesque bottoming out or eccentricity of the new implants. Needless to say applying a mastopexy is borderline with an atrocity in such cases. A multipoint capsulorrhapy in your case is a must, so do research well on surgeons with vast experience in breast revisions, bottoming out, symmastias, etc... do not accept any kind of breast lifts, snap-surgeries or minimal and simple procedures.
Do please keep in mind breast implants are NOT and should not be chosen based on such ridiculous paramaters as cc or gr, only the three dimensions sizing count in order to make the prosthesis a perfect fit for your frame. Finally, let me recommend latest generation anatomical shaped implants, macrotextured and cohesive gel filed models, a bit more expensive but undisputably and neatly superior, in subfascial plane is my choice.

It is hard to say without an exam.  If the breasts are a bit saggy now, then likely they will be more saggy with smaller implants. Again, an exam in person is important.

You can undergo an implant exchange which would also involve revision of the breast pocket.  Discuss with your surgeon including financial responsibility for the procedure.

I am sorry to hear about your concerns after breast augmentation surgery. Yes, you should be able to undergo downsizing of breast implants. Keep in mind as you are thinking about your goals that sometimes adjustment of the breast implant pockets (capsule adjustments) and/or breast lifting may be necessary to achieve the outcome you are looking for.   Whether or not the adjustment of the breast implant capsules or breast lifting will be necessary will mainly be dependent on your physical examination; specifically, factors such as the amount of redundant skin, quality of skin elasticity,  and how much you downsize will be important.  
Generally speaking, the best online advice I can give to ladies who are considering revisionary (downsizing) breast augmentation surgery is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "down a cup size” or "C or D cup" etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
3. Once you feel you have communicated your goals clearly, allow your chosen plastic surgeon to work with you in determining the best plan to achieve your goals.
I hope this, and the attached link (dedicated to revisionary breast surgery, specifically downsizing breast augmentation surgery concerns), helps. Best wishes.  

Thanks for the great question. You can definitely get small implants. The surgery to down size will be a little more involved because you may need to have the existing implant pocket modified a little bit to securely fit a new, smaller implant. I recommend seeing someone with a lot experience with breast revision surgery. Best of luck!~Dr. Sieber