The issue with subglandular implants is the likelihood that superiorly they will be more visible with time, and the risk of capsular contracture is higher. However, you have enough breast tissue, that a subglandular position looks good at 5 months, and you can get a nice result with a lift alone. Changing the implants to below the muscle will not increase cleavage, and adds some complexity to the operation. In other words their are trade offs to each. Leaving the implants in the subglandular position simplifies the operation and makes it more predictable in the short term. Long term the chances for maintaining an optimal appearance with a soft breast are better by changing the implants to a submuscular position.
I think that you would benefit from a small lift. It probably isn to necessary to change the position of the implant unless it is visible( rippling) in the upper pole.
From your photograph your result looks quite good to me. You will have better cleavage with the implant remaining in the sub glandular position. It will take many months for you to stretch out the pectoralis muscle to make for a nice result. In my opinion, a smaller peri-areolar breast lift would be as much as I would recommend. Your surgeon has a good plan for you good luck with your revision.
Based on your narrative and pictures, I don't know why you would need to reposition your implants. You do appear to need a bit of a breast lift, however, From your pictures, I would think that a periareolar (aka, donut) or short-scar (aka, lollipop) lift would work well for you.
I'm sorry to hear that you are having to have another procedure so soon. Your plastic surgeon who has examined and advised you most likely knows best how to proceed. Typically an implant in the subpectoral position is not as visible or noticable as one above the muscle. Therefore, they usually look a little more natural, but provide less cleavage. The photos provided do look like you would benefit from a lift. I don't hear any compelling reason to perform a pocket change to under the muscle at the same time from your question above. Discuss these concerns with your plastic surgeon to ensure that the two of you are on the same page before you proceed. Best wishes,
Erik Hoy, MD