We would have to reset all votes and then people would have to go back and re-vote for those they want. That would actually be a good thing as there are some dormant votes on there that likely don't need to be. The more I think about this, the more I think something in the 5 vote range would be ideal.
But yes, we would need to make sure the chain continues running while the unvote/re-voting process takes place... for that perhaps we just keep the same top 20 in there for the first 24-48 hours until there has been some time for votes to be recast and a new top 20 set?