Creating a new pocket for implant exchange?

Thank you for the question.  Although it is possible to have breast implants replaced in the sub glandular position, I do not think that it is in the best interests of most patients to do so.  Generally, breast implants placement subglandular position can be problematic;  concerns such as rippling, palpability, encapsulation,  accelerated "sagging" of the breasts…may arise.
In other words, if at all possible, I would suggest keeping the breast implants in the sub muscular (dual plane) position;  this is assuming that an aesthetically pleasing outcome can be achieved with adjustment of the breast implant capsules…In the end, exactly what operation you should undergo and what type/size/profile of breast implant should be utilized will depend mainly on what you are trying to achieve (in other words, what your goals are).
 You are correct in that during downsizing of breast implant surgery (sometimes) adjustment of the breast implant pockets (capsule adjustments) may be necessary to achieve the outcome you are looking for. Breast lifting may or may not necessary.  Much will be dependent on your physical examination;  specifically, factors such as the amount of redundant skin, quality of skin elasticity will be important,  and how much you wish to downsize.
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 helps. Best wishes.  

There are many reasons for creating a neo-pectoral or new pocket between the muscle and the existing capsule to place a new implant. Most of the time the procedure is performed when there are significant breast asymmetries caused by implant displacement or deformity. Sometimes one side will displace more laterally or inferiorly (bottoming out) or medially (creating symmastia or bread-loafing). Creating a new pocket can allow replacement of an implant into a more attractive or symmetrical position. 
Most of the time, however, a formal mastopexy is required in order to reshape the breast around the smaller implant. Of course, there can be many opinions about the best treatment. Please select a surgeon board certified by the American Board of Plastic Surgery and preferable a member of the ASAPS (American Society of Aesthetic Plastic Surgeons).