Autonumber field increments when test class runs and note that in Spring14 salesforce has released an option not to increment when test class runs.
Develop | Apex Test Execution | Options..., selecting Independent Auto-Number Sequence allows one to mostly avoid the gaps in auto-number fields when running tests
But I forgot to check the option and now there is gap in auto number field in existing records. If its a custom field I would have followed the steps from knowledge article but it is Standard Order Number field on Order Object where I don't see an option to Change Field Type to Text when I click on Edit next to Order Number
Below is the screenshot I see when I click on Edit next to Order Number.