What is the best way to separate financial results pre- and post-acquisition?

We have been acquired as of 9/30 and need to separate our financial results pre- and post-acquisition.  We currently have a ledger called ACTUALS which involves a calendar fiscal year with monthly buckets.  If we need to separate the ledger activity from after the acquisition until the end of the year, does anyone have any recommendations for a good way to accomplish this?  We would like to retain drilldown capabilities on all the data and would prefer not to have to modify or clone too many of our existing reports to reflect either pre- or post-acqusition information.