I would say at least half of the posters there do not have their abstract on it. The poster is, itself, sort of an abstract. So to me it seems like severe overkill to put a separate abstract on there. Nobody cares. They aren't going to reject your poster. I have seen people win Stowell-Orbison awards who did not have the accepted abstract (or any separate "abstract" on their poster). I think they might tell you to put it on there, but unless it says something that you aren't saying elsewhere, don't bother.
As far as changing the text, most people do this also. Data changes or is added to between the time of abstract submission and the actual meeting (which is something like 6 months). You may get some questions if your data or conclusions are different from what is posted in the Modern Pathology issue or online, but people do it all the time.
It is also fine to add more authors. People don't really care. All that matters is that your abstract is accepted as a poster, you are assigned a poster number and time and date to present it, and you show up with your poster at that time to present it.