I previously submitted an abstract to an oncology specific conference involving a retrospective review of patients which was accepted. Since then I have had the opportunity to add a few more patients and have now finalized my dataset. This hasn't resulted in any major changes to my findings, other than giving a better sample size for the population being evaluated, slight changes in Hazard Ratios, etc. Given the fact that my overall findings haven't changed, is it acceptable to present the final findings during my poster presentation instead of the data from the original abstract?
Yes, you should present the latest and most complete data. I doubt that anyone would be surprised by this or object to it.
However, in the alternate case, where later data shows the earlier results to be invalid, one should bring it to the attention of the conference committee and ask for guidance. It might be especially important to present such findings, though they might also indicate some important error somewhere.
But in your situation, yes, show the best available data.