Sounds like you need to speak with a probate attorney, first off.
Generally speaking, so long as your grandfather is competent and not under duress, he can change his will whenever he wants.
If you are a beneficiary under the will you will get notice that the probate has been opened. But it would make sense to talk to your own probate attorney ASAP once your grandfather passes in order to monitor the courts.
I would certainly recommend that you exhaust your informal channels to obtain the will before resorting to the courts. If you can't get it, however, there are some things you can do. A person in possession of a will is required to lodge it with the court within 30 days of the decedent's death. If they don't do so, you can file a petition for an order to compel them to disclose it. Once the will has been lodged with the court you can review it. If something looks fishy you can then file a will contest. You'll have to show that the will was the product of undue influence or that your grandmother lacked capacity when she executed the will.
Having handled a number of these cases, I can tell you that it will be difficult to handle this without the assistance of an attorney. You should start by getting your hands on the will and any prior wills to see if any major changes were made in your grandmother's will. If you see something that doesn't look right, get an attorney who practices probate law and litigation (these cases are more litigation than probate).
Best of luck to you.
This advice is based upon the limited facts you have provided here and is not meant to be comprehensive or to replace an actual consultation with an attorney.