When you create your Data File, Big Business warns you to check the Start Date and Year End Month carefully because once Setup is complete they cannot be changed. This does not prevent the IRS or your Board of Directors insisting that you change your Fiscal Year End Month. (3021)
One remedy is to create a new Data File. You can retain most information, except historical transaction data, by using exports (Save to Disk, in the various QuickSearch tools) to create your new data file. This is a good option when a change like incorporating changes your accounting, too.
If needed, we can email a solution to change the month, and nothing else. This disrupts Budget reporting, but is otherwise quick and painless. Contact support@ bigbusiness.com for details.