This is a challenging exercise in post merger integration. Most often than not, acquires have its existing accounting system. I guess the integration of financial systems must also be risk based. Is the current accounting system of the acquired not reliable or not up to par based on the internal control standards of the acquirer? If yes, then a robust transition plan must be in place. Otherwise, you will continue to doubt the data which will be a hindrance to overall post-merger synergy harvest.