I believe Kickstarter actually won't let you withdraw a pledge if doing so would drop a funded project below its funding threshold.

edit: found the rule (https://www.kickstarter.com/terms-of-use),

Backers may increase, decrease, or cancel their pledge at any time during the fundraising campaign, except that they may not cancel or reduce their pledge if the campaign is in its final 24 hours and the cancellation or reduction would drop the campaign below its goal.