Just curious - do you mean that the immigrant visa is stamped in your passport? Or do you have an I-551 stamp (temporary evidence of greencard), from previously entering the US using the immigrant visa? I thought that immigrant visas were in the form a large envelope containing your file, with a one page visa attached to it (that may not be the case, I'm just going on what I recall from several years back. In my understanding they're one-time use, effectively converting to a greencard on entry to the US).

In any case, even if you already have the I-551, I'm don't know how that would be handled by the US if your country of citizenship subsequently changed.
I agree with the advice to talk to an immigration attorney. Good luck.