Is it possible just to exchange my implants without lift? (photos)

Yes it is possible #downsize to smaller implants and also alter the positioning; if possible. However, a Breast #Lift to #tighten loose skin is usually recommended to better optimize the end result. I prefer to use a #shortscar technique, #LollipopScar or #DonutLift” rather than the majority of surgeons in the United States that use an anchor pattern lift which involve more significant scarring. Other factors which contribute to the outcome are #size, #shape, and #profile.  The #profile is determined by the amount of #projection versus roundness that the you desire, as well as the #size of the implant and size of your chest. Also, the placement or incision location will also have an effect on profile.  More women tend to choose silicone breast implants due to the natural feel and appearance. Silicone implants are now made of a cohesive gel which are anti-leak even when cut. Look and feel like natural breasts due to the cohesive silicone gel material. New generation silicone gel implants are far superior than those used in the past and have a proven track record of safety. Silicone implants have gone through more testing than any other medical device and are the preferred choice for over the muscle implant placement. Also, the right implant for the revision is contingent on shape of body and your desired outcome. Another element to a satisfying result of your #augmentation is realistic #expectations. Talk to your surgeon at greater length and be sure to view before and after pictures and reviews beforehand to gain more reassurance of the procedure.

The answer to your question is that a modest reduction of implant volume will not lead to the need for a lift. My concern is that your dream pictures are in garments and I am not clearly seeing a difference between what you have and what you want.This decision would require a formal conversation with a board certified plastic surgeon to clearly understand if what you are asking can be achieved.The procedure you mention is very reasonable and can be done. The bigger issue is if it is the right thing to do for you. Smaller breasts with mild skin laxity may not be the right path for you.Seek out a board certified plastic surgeon to evaluate you and discuss these issues in more detail.Good luck and I hope this was helpful.

Your questions are the exact same questions we plastic surgeons ask ourselves every day.  Unfortunately, your skin elasticity, propensity to scar, possibility of capsular contracture, and other future events cannot with certainty be predicted in advance.  We can however control risk based on what we know. 
If you want the breasts to come together in your mid-line as they now do, choose implants that are the same width.  Volume (450, 500, 600) matters less than dimensions (width and projection) of your implants in this respect.  A high profile (high projection) implant that is smaller volume than another high profile implant (600 to 450) will be smaller both in projection and in width.  To keep your breasts together (width) and go smaller (volume) requires either a lower profile device (less projection but same width) or lateral capsulorrhaphy to move the smaller high profile implants toward your center.  That latter maneuver however may position your nipples eccentrically on the breasts, and would mandate post surgical compression bra for a period of time to prevent implants from lateralizing.  I would also recommend textured surface devices if you insisted on narrower high profile devices and "cleavage," and you insisted in sub-pectoral positioning, because if they're smooth surface, your muscle may tend to lateralize them over time by dynamic compression of the devices against your curved ribs, and by gravitational influence.  
I like avoiding a new (infra-mammary) scar when possible, but re-entering the areola region may pre-dispose you toward capsular contracture, so this is a decision between you and your surgeon.  Replacing implants through the existing areola scar doesn't obligate you to another breast lift.  Also, if you already have the areola scar, re-doing an areola lift may tighten your skin envelope around smaller devices without any new scar.
Discuss all of these with your surgeon.  You look pretty good now, sometimes, adjusting to your perceived imperfection (adjusting to the known) is preferable to embarking on a change with uncertain outcome.