Think about this differently and remove systems.
Ideally you would want the shop floor, if possible, to gear production in advance of the cutover in a manner that enables stocking of fast moving items where possible.
My advice would be to run production down, do not load production orders you will not start before the cutover. For those in WIP close them at the cutover, finish them short, consume and end to get a final position. Then decide if the balance remaining is needed .
You want the management of this to be as simple as possible. If you start trying to take over uncontrolled orders in process you need to consider where they are, the work involved and the stock position of the raw materials and finished goods, all of this has implication on the financial reconciliation and the stock reconciliation.
Try to keep this area as simple as possible to enable an easier cut-over.
If you do this you bring over very few, recreate ones you need (or run planning to get new suggestions) and then you are only importing ones at a created stage, or you are manually keying them depending upon volume.
