Can implant exchange increase capsular contracture risk?

Replacing your implant with a smaller one in the same pocket should be fine if using a gel implant. If you are thin with little breast tissue, you will want to avoid a saline implant in the subglandular position to avoid rippling. With regards to capsular contracture, you should be fine keeping your implants in the same pocket if you currently have soft breasts.
Realize, however, that anyone with silicone implants can at some point develop a capsular contracture. There is evidence to show that the submuscular position has a decreased incidence of capsular contractures but this may not be your best option at this point and given that you are a personal trainer.

#Capsular #Contracture is a condition in which the capsule around the implant thickens and squeezes the implant making it hard and often changing the shape and position of the implant. It is more common in nicotine users (e.g. smoking or nicotine patches or gum). Effective treatment of the capsule frequently involves completely removal of the capsule (capsulectomy), and occasional incisions in the capsule may be somewhat helpful (#capsulotomy). In the past, closed capsulotomy, in which the breast is squeezed until the #capsule breaks, was used. Currently, most doctors avoid this technique because of complications associated with the technique and because the implant companies will void their warranty if this technique is used. 
At times it is necessary to change the position of the implant, either above or below the muscle, depending upon the initial position, changing the #implants themselves and, on occasion, the use of Acellular Dermal Matrix (ADM) may be required to prevent continued problems. Although the published risk of capsular contracture is approximately 9-11%, in our practice it is somewhat lower after initial implant placement. Once capsular contracture occurs, the risk of problems with secondary surgery rises to between 25% and 40%. The use of #Singulair and Vitamin E may be of benefit. If you had a Sientra Textured Implant placed as a primary procedure and a contracture occurs in the first two years after primary augmentation, then their implant warranty may apply.
I suggest consulting with a board certified plastic surgeon to discuss your best #options. Plastic surgeons who have specialized in breast surgery and cosmetic surgery are suitable to perform your breast implant removal or revision. Aside from checking board-certification, it is suggested that you look at before and after photos of the surgeons actual patients, and read patient reviews. Gathering all of this information will help you make a well-informed decision.

Removal and replacement of
breast implants should not increase the potential for capsular contracture in
someone who has had no previous history of this problem.Since you have not had problems with subglandular
implant placement, there’s no benefit to moving implants to a submuscular
position.In fact, this maneuver might
be associated with an increased surgical morbidity.In some cases, it might be necessary to perform a
capsulorrhaphy to decrease the breast pocket size when smaller implants are
used.If you’re considering removal and
replacement, it’s important to discuss these issues with your plastic
surgeon.Your surgeon should be able to
formulate a treatment plan that addresses not only your anatomic findings but
your aesthetic goals as well.

I would think carefully about replacement of your implants.  It is possible that you could develop capsular contracture in spite of your good result the first time.  In addition, all the other risks of surgery are inherited.

Exchange of implants is most easily performed in the same plane.  I would recommend maintaining the same implant pocket and repositioning under the muscle if there is significant risk of capsular contracture.  There are no immediate reasons for you to replace the position to subpectoral. 

If you have had success with your current implants in the subglandular position, then you should be fine to have them removed and replaced in the same position. However, if you have had problems with implants in the subglandular position (i.e. visible or palpable rippling, capsular contracture, unnatural appearance) then having them moved to the sub-muscular position is a good choice. Implants in the subglandular position do have a higher risk of developing capsular contracture in general. This is one of the reasons that I generally recommend placing implants below the muscle. But if you have done well with your implants in this position, then I would leave them there.