Changing the range of existing pre-assigned bidder numbers and setting or moving individual bid numbers can be done from a single place that requires a password for extra security.  

Specify the starting number/range for future tickets sales, at Admin > Gala Auction > General Settings > Minimum New Guest Bidder Number.

If your site is set up to assign bidder numbers at check-in, the temporary numbers (muted gray) shown are not editable and must be assigned at check-in.

Note:   A bid number cannot be changed as part of a complete reset of bid numbers after the guest is checked in.  The system will allow you to reset a checked in guest's bidder number as an individual edit - but USE EXTREME CAUTION if doing so during your event (make sure the guest has not yet placed any bids.)  

You can also "undo" check-in (if you are certain the guest has not placed any bids yet with the existing bidder number):  Tickets > Edit Guest > Uncheck the "Checked in" box > Update User.