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Not just AS/400, but any decent accounting program should prevent you from doing that.
Closing a period means no more transactions.
You might look for an "unclose" option, but that would be unusual.
I Googled DMAS AS400 and found to hits. One was on the IBM website indicating that the product was out of support in 1993!. The other was w/ a company called I/O International. They indicated that the latest enhancements were in 2012, but it does not appear that the website has been updated since then. Appears that this is an accounting package specialized for the distribution industry.
The issue of replacement software is seperate from the hardware issue. There are several software packages for any given hardware system. The question is really which package will serve your companies needs best. While I (and I am sure others in this forum) have preferences for hardware based on reliability, ease of support, etc.
A given software package may peform very well for company A, and still be very wrong for company B.
What do you want the package to do? Just accounting? integrate inventory? Manufacturing? Shipping/Recieiving? etc. For very large companies to compile the list of requirements can take a couple of years. The more defined the requirements, the better the match, the happier you will be with your selection. (There are books written on this topic).