Merging 12 Business Units into 6 new Business Units. What is the correct setups and procedures

We have 12 business units that need to be merged into 6 new business units. We are currently utilising AR,AP, GL, CM, EPM and need to understand what is the process of disabling the Old Business Unit so that no more transacting can occur on these older BU's once the new ones are created. The plan would be to move balances from the 12 Business units to the newly created 6 business units and continue transacting on the new business units. Also if we need to see historic data how can we retrieve this data if we disable the old