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 Site Settings > Events > Gala Auction > Manage Bidder Numbers.
If your site is set up to assign bidder numbers at check-in, you can click the Assign button next to a guest's name to assign them an bid number. Be aware that doing so will automatically check in that guest.
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.
To reassign existing bidder numbers:
Go to Site Settings > Events > Gala Auction > Manage Bidder Numbers
Note: Reassigning in bulk requires Chair role.