Hello Mathew,
Installing Ledger V9.0A allows you to keep V8.1A installed on your computer, but your data cannot be accessed by the older version once it is converted. You would have to make a copy of your data and "point" the 8.1 version at the copy. I don't know whether that corresponds to your definition of "functional" for your accountant.
I cannot imagine why the accountant would prefer the older version. The functionality he/she is likely to use has not changed much in the newest release. It's just easier to use, especially over the year end.
The subsidiary ledgers will happily send their information to the new version. The accountant should check to make sure they are happy with the information being transfered, there are lots of new options that can be sentwhen the new general ledger is available.
Thanks for using Adagio!
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Andrew Bates