I really don't mean to toot our horn, but you really, really need to read the titration section of the BR general chemistry books. Look at the areas the curve is broken into. The buffer zone is a natural part of the titration curve for any weak acid or weak base. So a buffer would not affect a titration curve per say; it is a part of the titration curve.
In the beginning you have whatever you added. The pH will change rapidly as you add titrant, because it is predominantly that species in solution at the start and you are depleting that species quickly. But over time, you start to get more balance in terms of the concentrations of the two conjugates, so the system is a buffer. As you continue to titrate, you will eventually deplete the initial reactant, and it will no longer be buffer. When this happens, the pH will rapidly change as it approaches the equivalence point. Once past the equivalence point, you are just adding a strong reagent without any reaction occurring, so the pH will approach the pH of the titrant. Those are the areas of a typical weak titration curve.
Is your question about what happens if you titrate a buffer? If so, then rather than starting the titration curve at the typical start, you now start it in the middle of the buffer region, where pH = pKa. You are in essence doing a half-titration in this case. Again, I really don't want to sound like I'm promoting, but the BR general chemistry books explain this so much better than every other book (including text books).
