How can complexity be increased or decreased in AES?
This begs the question, why would you in any real-world circumstance wish to reduce the difficulty for an attacker to break your cryptosystem?

To answer your question practically, the only reasonable way I can think of to accomplish this is to simply reduce the entropy in the key. At 100%, all 128 bits of the key are used. At 50%, 64 bits of the key are used, and the rest set to some static value (e.g., 0). At 25%, 32 bits of the key are used, and so on.

That said, I want to reiterate that there is no reason I can possibly conceive of to support such a feature, outside of sheer curiosity.

Edit: Note that this will result in an exponential decrease in security, which maps well onto how cryptographers typically think of the security of a system. Modern ciphers like AES-128 so strong that a 100-fold reduction in difficulty to break still leaves brute force well outside the realm of conceivable human feasibility ($\frac{2^{128}}{10^{2}} \equiv 2^{121}$).