Sorry for the late response, but just in case you are still looking for a solution I'll go ahead and post. There's no direct way to correct the accounting periods but there are two options other than restoring a backup.

The first option is to do a company rebuild. Detailed instructions are in the help and in the knowledge base at  How do I rebuild a company? . The basic idea is that you export everything from the original company, then set up a new company with the correct accounting periods and import everything into it. For very simple companies it's feasible for some people to do it themselves if they closely follow the instructions. But the more complex your company, the more likely you are to have problems. Since it leaves your original company intact, the only thing you risk by trying it is the time you invest.

The other option is to work with a certified consultant (like me) who provides data repair services. The periods can be corrected without having to re-enter any transactions. The fee varies depending on the turn-around time required. If you would like details you can contact me at [email protected] or 918-851-9713.