Converting history has never proved to be cost effective for our customers. That doesn't mean that every customer doesn't ask for it in the beginning. Since we do all our work on a fixed fee basis, we tell the how much it will cost, before we get started. The less we know up front, the higher the quote to address the risk. This allows them to make a better ROI decision.
As an alternative, we typically set up a VM and move a copy of their current accounting system to the VM to preserve the OS, app, and data for research and lookup.