Is it possible to go under the muscle with saline after going over the muscle with silicone?

The problem with saline over the muscle is the tendency for more rippling of the implants.  I always put saline under the muscle. If you currently have silicone and want to switch to saline, it is easy to convert them to under the muscle.

Yes,
you can change implant type and position but I always recommend small round
textured silicone gel implants placed retro-pectoral since they look and feel
more natural, are more stable, less likely to ripple or have complications needing
revision. Implant profile is irrelevant
in the retro-pectoral position since the muscle compresses it. Unfortunately, you have not told us your bra
size but if you are 32 or 34 each 100 cc’s of implant corresponds to 1 cup size
change. If you are 36 or 38 each 200
cc’s of implant corresponds to 1 cup size change. 550 cc to 650 cc implants are very large,
will extrude inferiorly and laterally requiring revision. 22% of women who undergo breast augmentation
alone require revision within 2 years because they are dissatisfied with the
result. A frequent cause of
dissatisfaction is over implantation. Best Wishes,Gary Horndeski, M.D.

I agree with both previous posts.  In general, when saline implants are placed in front of the muscle, in a tight skin person with little breast tissue, they will look more fake than the silicone implants.  If you have some breast tissue, then depending on the size of implants, it will depend.  The larger you go, the more fake the look.

   This is difficult to assess how something will look without pictures or an exam.  Transitioning to a different plane is possible and a submuscular augmentation does tend to provide more coverage and a more natural appearance.Kenneth Hughes, MDLos Angeles, CA

Thank you for the question.To best help you, a physical
examination or picture evaluation is required in order to determine
which procedures and techniques would be best for your case.Dr. Campos

Implants can be changed from on top of the muscle (Sub glandular) to behind the muscle (Sub muscular). During the operation a submuscular pocket can be created and the implant is replaced. Placing the implant behind the muscle puts more tissue on top of the implant. In general this will give a more natural result. There are many factors that affect whether implants look natural or fake. These factors include if the implant is beind or on top of the muscle, how much breast tissue you have, size of the implant, and saline vs silicone. Although it is difficult to say without an examination, I would guess that 550–650 mL saline implants on top of the muscle would likely look fake. 

It is difficult to say without a proper exam in person. What I will say is that the larger the implant and the less native breast tissue you have, the more fake they will look.

Looking fake is not a function of in front or behind the muscle but the volume you pick relative to your to your skin enevelope and body size.  Large implants generally look fake is small women with poor breast and skin coverage whether in fron or behind the muscle.Dr. Corbin

Thank you for your question. Removing over the muscle silicone gel implants and replacing them with saline implants placed beneath the chest muscle is commonly done. Be aware of the problem of rippling of saline implants and discuss this with your surgeon. Over inflating the saline implants and placing them under the muscle can help reduce this problem. However if you're very thin with very little breast tissue and little body fat the risk is increased.Please see a plastic surgeon who is certified by the American Board of Plastic Surgery, experienced in breast implant surgery, and who has an excellent reputation in your community.