Merging with another company

We are merging with another company and need to come up with a way that we can report as company X up until the merger and as company Y the day after the merger. We do not yet know if we will be staying with Lawson or moving to the finanical system they have, right now we just need to figure out a way to have a clean cut off between the two companies and report as Y company going forward. We currently have 23 active companies and we have subsystems and external systems that interface into our current company structure and do not have the time or resources to change these programs. After we merge we will still have 23 companies and will keep the company numbers the same so the interfaces still work. What I am finding most frequently as the option recommended to use is GL110, however, we have a lot of data in one of our companies and doing a copy to a new company with data, delete the old company, and recopy structure only to the new company will require that our system be down for about 48 hours, which may not be an option if the "go-live" day is a Tuesday or something. Has anyone been through this and if so what process did you use.