Is that all? I thought it was going to be something really hard. Just go back into the setup options and change the month back to July.  Also note that it is advisable to NOT close the month right at the end but to give it a good week to make sure everything is entered. MAS errr Sage 100 ERP has two periods, current and future so you can post way into the next as you get all the business for the current month reconciled. 

You can go back into the module and re-open the previuos month in modules on the business framework as you no longer have the summary tables as before so actually very little is done at month end but don't tell anyone I told you.

Thank you BigLouie you're awesome!!

Please note that you are in IT so please understand that you should rarely if ever close on the first day of the month. There is always something that needs to be posted a few days later. Plus you need to balance the modules to the GL and make any adjusting entries or research any issues. Lots of time you get your bank statement right after the first so you need a few days to do bank rec for the previous month. As they say, there's always something.

I have just did the same thing... mistakenly closed the PO period prematurely (close Oct on Oct 9th!). I change the setup options and change the month back.

I am hoping that everything can be processed smoothly. If it works, do I re-close the period (October) again in early November?

Yes, close the period as you would normally.

Generally speaking G/L is about the only module I would recommend setting dates back like that.

In 4.3 and prior (for the P/O module),  setting the date back can really impact some reports due to the data structures.   I haven't really tested it in in later versions.   It be OK in the later versions.

Definitely,  DO NOT do this in payroll.   It will a huge mess!   You restore from backup.