Can elections be stolen and rigged? Sure. However, falsifying voter registrations in bulk to influence a result is not only real iffy, I’m not even sure it can be done. Here’s why.
1) The voter registrations would need to be tampered with continually, starting early on. Yes, many state voter registration systems are ancient. They also have safeguards, audit trails, lots of people watching, as well as other security measures. A friend who works for the federal government can access sensitive information on a need-to-know basis. If he accesses data he doesn’t have permission to look at (like by entering an incorrect ID number), he gets an email quickly from security asking to explain what happened and why. This is on a paleolithic COBOL system. Modern software is considerably more secure.
So, no, someone can’t hack into the New York State voter registration system and flip 100,000 voters to independent from Democrat without someone noticing, and quickly.
2) Most important, how could anyone possibly know what to change registrations to? Let’s pretend I am a Devious Hillary Operative who has skulked in the New York voter database. There is no way to know if I flip 100,000 Democrats to independents that they were going to vote for Sanders not Hillary.
All of this is kind of a moot point, because it appears Hillary has stomped Sanders in New York. I doubt there was any appreciable voter fraud by anyone.