Witness votes are permanent. Each member has 30 votes and it is very important for the decentralization of the blockchain that we use them. You can withdraw your support for any witness you have voted for at any time, but it isn't necessary to re-vote once a vote is cast..

Witnesses not in the top 50 can be typed in manually at the bottom of the witness page here:
https://steemit.com/~witnesses

You can see who is supporting our witness and who has withdrawn their votes here:
https://steemdb.com/@idnit/witness

And you can see information about all witnesses here:
https://steemd.com/witnesses

Thanks for your support !