Is it worth swapping from 300cc to 400cc? Would there be a massive difference or would the pain not be worth the change?

Thank you for sharing your question.  Unfortunately without a full series of before and after images it is difficult to offer definite advice but in general 100cc is the minimum size increase to have a noticeable change.  That said if you have a smaller frame and more narrow chest 100cc may make a large difference compared to a woman with a wider chest.  Talk to your surgeon about you desires, they can best help you make a decision. 

Most women need at least 100cc change in implant size to see a visual difference in their breasts.  I generally recommend increasing 150cc if you want to notice a difference.  In laymen terms, 150cc is 1/2 a can of soda so use that as a frame of reference.  Most implant exchanges are much easier recovery than the original surgery with minimal pain.  I hope this helps.

Going up 100cc will likely give you a noticeable change in your appearance. If your surgeon stays in the same pocket the pain is usually minimal after surgery because the pocket has already been made from the previous surgery 

We feel that in order to have a visible difference when up sizing a breast implant we need about 150cc to make the effort "worth it". Certainly the change from 300 to 400 will not be massive as you will have to set your sights higher.

Since you are only increasing in 100 cc's, this is not a massive difference and may not increase a full cup size, depending upon your original size and frame.  However, the pain would probably be much less since the tissues, especially the muscle, would not have to be stretched as much as in the initial surgery. Your surgeon can best guide you to what size you may achieve.