7400s can do of things if the memory of not initialized correctly. Eg revert to an old config after a powerfail, but not of you manually power it off.

The programming is stored in the mp40 card.

You could have just shutdown the 7200 cabinet and replaced it with the 7400 still powered on.
Unlike the old dcs it doesn't seem to have an issue with it.

If you have auto mmc backup turned on you can try restoring from that