I'm not a lawyer, and the facts of any particular case are determined by its specifics. Thus, you won't get a definitive answer here since you can't disclose your invention. That said, I have a few comments on your question.
If your invention is something new, then it isn't mentioned in the previous patent. They can't just add new claims that aren't supported by the body of the patent. Thus, I think it highly unlikely that the owners of the granted patent could somehow get their patent re-examined (or file a continuation) and extended after seeing your invention. You product (and patent if you obtained one) in this case would be prior art.
What I should point out is that if the previous patent contains the basics of your invention and you are merely improving upon it, it is very possible that you could receive a patent for your improvement, but not be able to sell the product it since it might still infringe on the previous patent.
As I said, you can't post the specifics of your invention before filing so there is no way to answer your question definitively. Thus, the only prudent action would be for you to actually consult with a patent attorney who could guide you to not only patentability of your idea, but also whether you will have freedom to operate. Again, I'm not a lawyer and this isn't legal advice.