Your SP is yours to keep. If you "power down" your Steem Power is converted to Steem over a 13 week period and then you are free to sell it. Only your voting power goes up or down based on how often you vote. The size of the upvote reward is determined by how much steem power you have AND your current voting power.
Resteeming is free, it costs you nothing.
Cool. Thank you. :)