Is a 75cc increase a noticeable amount with a implant replacement from saline to silicone? Should it be more?‏ (photo)

Thank you for the question and good quality photographs!  Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with revisionary breast augmentation surgery.  Some thoughts may be helpful to you: there is no specific  implant “life”;  in other words, if patients are pleased and there are there have been no problems with the breast implants there is no necessity to remove/replace.  Sometimes, patients will have very nice/soft outcomes with saline breast implants;  consultation with well experienced board-certified plastic surgeons will help you when it comes to determining whether the use of silicone gel breast implants will necessarily be “softer”.  For some patients, especially petite patients or those with smaller breasts implants, a change of 75 cc may result in a very noticeable difference; for other patients a greater increase in volume may be necessary to achieve the desired outcome.
Generally speaking, the best online advice I can give to ladies who are considering  revisionary breast augmentation surgery (regarding breast implant size/profile selection) 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 "C or D cup”  etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate. Again, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size.
 
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this (and the attached link) helps.

I
recommend a new technique called Implant Exchange with Mini Ultimate Breast
Lift™.Using only a circumareola
incision it is possible to reshape your breast tissue creating upper pole
fullness, moving them high on the chest wall and more medial to increase your
cleavage.At the same time, implants can
be exchanged.Aligning the areola,
breast tissue and implant over the bony prominence of the chest wall maximizes
anterior projection with a minimal size implant.Small round textured silicone gel implants
placed retro-pectoral 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.This technique avoids the ugly vertical scars
of the traditional technique, maintains nipple sensation and the ability to breast
feed.Best Wishes,Gary Horndeski, M.D.

This may be more than you want to consider but some of my patients who REALLY can't decide elect to have their saline implants evacuated in the office prior to surgery. The advantage is that they can see exactly how much breast tissue they have before committing to a particular implant. Its relatively painless except for the pain of walking around without a breast for a couple days. Not for everyone but always an option...Good luck