What do you mean? MIT is essentially as open as you can get. The worst that can happen is that they will relicense, eventually, to force big users to pay, but when that happens everybody knows how it goes: some consortium of other big companies forks it and continues development as if nothing happened.
Step 1: discontinue the public repository, step 2: sell access to your GPL codebase.
The GPL (and even the AGPL) doesn't require you to make your modified source code publicly available (Debian explicitly considers licenses with this requirement non-free). The GPL only states you need to provide your customers with source code.
Sure, but it also allows your customers to modify the source code you provided, and distribute/sell it. With MIT they can simply relicence it and sell binary-only versions. The open-ness stops at that point.