You can't. That's the thing. He didn't just switch it off last time. It had to be removed. It's not an easy thing to do in either direction, from what I've been able to find out. We think it might have happened during the time he wasn't in the dome. Some sort of weird thing with the dimensional stabilizer.
[She didn't know about the new memories coming in overnight or any of that, but she did know that some people who disappeared would come back with new memories, and that it could be related.]