Strictly speaking, no. MIT does not give you permissions to change the license. But you can incorporate MIT licensed code into a project that contains code that is under a different license of your choice, open or closed source for a nearly identical effect.

So you can just license all future contributions under AGPL and use the MIT licensed code, effectively making the combined work AGPL. But the MIT part remains MIT, anyone could fork your project, remove all AGPL code and return to MIT. It’s wise to clearly mark which code is which, especially since MIT requires that the copyright notice must be retained:

> The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.