@peyre proposed in issue #246 that

Incidentally, I was thinking along these lines re. allowing the player to downgrade the duration level of a duration mission. On duration tests (and any mission that you've assigned a higher duration level to than it requires), an extra button might appear in Mission Control. The reason I think this would be important (if it's practical to implement) is that duration tests sometimes fail. Say you assemble a Dur C and schedule a Dur D for next turn, and the C mission has to cut the Dur step short and you're still at level B. You're faced with the choice of flying your Dur D at a -5 penalty or scrubbing it. It would be handy if you could just change it to a C mission.

Having run into exactly this issue before, I concur. While there's uncertainty about the viability of downgrading a manned mission to an unmanned one (Issue #241), cutting the duration of a mission should be much easier; that's actually what happens when a duration step is failed during an endurance mission.

Additionally, I think we might consider setting the downgrade penalty differently in such cases. Say I've scheduled a Manned Orbital w/ EVA. If a problem with R&D or a news event produces a poor EVA suit reliability, I'd have to consider cancelling the mission, yet I'd loathe allowing the opponent to beat me to the Manned Orbital milestone. This is a good case for downgrading a mission; under these circumstances, a single risky element jeopardizes the entire mission.

However, this logic doesn't translate well to a duration downgrade. Lessening the duration to avoid a -5 duration penalty isn't so attractive if there's a -3 downgrade penalty attached. Sacrificing two duration steps makes more sense, because a -7 penalty is tough to swallow. But I imagine a single duration step of -5 is far more likely, and losing the prestige of the extra duration step (and 3MB) to prevent a -2 penalty is a much harder decision. In short, the relative cost is greater. It may be that, yes, we are mostly concerned about the case where the player is facing a -10 penalty, but it's worth consideration.