That's something you've always had to worry about though. You can't force a user to upgrade, so your best bet is to handle those issues server-side.
Sure you can. Netflix will sometimes have to do this. The old software just isn't allowed to talk to the server anymore.
I don't recommend that unless you have a mission critical bug, and even then I'd discourage it. A user is in charge of his/her device. I version APIs on the backend and then simply push out updates like any other software. If they choose not to use it then not much I can do. Eventually I'll turn the older API versions off and they'll either have to upgrade or stop using the app.