I'm 5'10, weigh 145 lbs, and I have 700 cc saline implants. My doctor suggested changing to 800cc silicone. Should I?

This question is difficult to answer without pictures.  My philosophy is typically against large implants in general due to the potential risks.

Thanks for your question,  an 800 cc implant would only increase you by less than 1/2 cup size.  Can not really tell for sure with out an exam or photos but you may want to consider a lift if you are trying to achieve more upper pole fullness.  

Thank you for the question.  In regards to specific size/profile of breast implant, despite good intentions, online consultants will not be able to provide you  with  meaningful specific advice.  Ultimately, careful measurements, dimensional planning, and careful communication will be key.
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're looking for. Ask to see lots of examples of his/her work.  2.  Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or 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" or "fuller" 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 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.  The use of temporary intraoperative sizers, viewing the patient's breasts in the upright and supine position, are very helpful during the breast implant selection process.
I hope this, and the attached link (dedicated to larger breast augmentation outcomes), helps.

No posted photo! So very hard to give an acceptable response for this issue you present here.....////

I am not sure why you want to exchange the implants..  Is it because of a desire to be larger, or is there a shape difference that you are concerned about?  Without pix this is difficult to tell about.  Good luck to you.Frank Rieger  M.D  Plastic Surgeon.com