I don't think you can change the IMSI remotely, or at least, you're not supposed to be able to. Most US MVNOs use IMSIs assigned to the operating carrier, which makes it hard to switch the operating carrier. Some do use their own IMSI, which you could then conceivably change network agreements and use a different network, but with today's networks, your users might have phones band optimized for the old carrier, and not have good reception with the new one.