You'd have to check with the DNC Rules and Bylaws committee.

One might suppose that "cheating" could result in sanctions being applied to the candidate -- loss of delegates, perhaps -- for violating the rules of campaigning. Fines, perhaps.

20.C.1b, DNC Delegate Selection Rules:

A presidential candidate who campaigns in a state where the state party is in violation of the timing provisions of these rules, or where a primary or caucus is set by a state’s government on a date that violates the timing provisions of these rules, may not receive pledged delegates or delegate votes from that state. Candidates may, however, campaign in such a state after the primary or caucus that violates these rules. “Campaigning” for purposes of this section includes, but is not limited to, purchasing print, internet, or electronic advertising that reaches a significant percentage of the voters in the aforementioned state; hiring campaign workers; opening an office; making public appearances; holding news conferences; coordinating volunteer activities; sending mail, other than fundraising requests that are also sent to potential donors in other states; using paid or volunteer phoners or automated calls to contact voters; sending emails or establishing a website specific to that state; holding events to which Democratic voters are invited; attending events sponsored by state or local Democratic organizations; or paying for campaign materials to be used in such a state. The Rules and Bylaws Committee will determine whether candidate activities are covered by this section.

That's all that's in the rules.

And you know, it'd be kind of hilarious if Clinton were found in violation for two closed-but-open appearances, and Obama's permission for national ads in FL retroactively taken away, rendering the FL delegates inapplicable to either candidate.