That is a lot of transactions to deal with. Too bad it took 9 months to discover the error. If I were in your position I would strongly consider an action that only addresses the GL implications of the problem and not worry about the other modules (with the possible exception of Cash Manager). Posting period is really just a GL critical item. If the AP, AR, PR, etc transactions have the wrong posting period but you fix the GL side then the only issue is that the various sub-module reports (like AP Transactions, PR Transactions) will still reflect the incorrect posting period but do not affect the GL (that you will have corrected. Other items, such as W2 information and 1099 information are not posting period driven so they will still be accurate.
The fact that you said this is 9 months implies that all the transactions are within the same fiscal year (though be it that fiscal year is 2 months off and has the wrong year id). It would be possible to create a query to "slide" the accthist values back 2 positions and to change all the GLTran posting periods back 2 (and, edit GLSETUP for the correct fiscal year). This would make the GL and financial statement properly reflect the fiscal year activity.
You can take the route you indicated in your last response and use the old database for detail and the new database for going forward. You loose some of the direct drilldown capabilities (without switching databases) but that may not be all that important to the client.
The risk in modifying the GL, and only the GL, is one; getting it done correctly and two; forgetting down the road that the first nine months will off by two months. And, of course, the fact that you would be moving backwards, 2 periods would be doubled up in the sub-modules (not in the GL). This last fact might be the most important factor in deciding what to do.
I mentioned Cash Manager as one sub-module that might have to be corrected because it ties cash activity from the various sub-modules to the GL and that has to balance in order to move on to the next month in Cash Manager. While most of Cash Manager is transaction date driven, it does still have to reconcile to the GL. I would have to think about how my idea would affect Cash Manager.