I made the unpleasant discovery this week that if you key convert any part of a project with billing or invoices, that Vision loses the billing terms and invoice history for whatever part was moved.*
It is possible to repair and recover this data, but not without some serious sql programming and moving data around to where it needs to go.
*note: what I mean by key convert is not just changing a project number or phase number, but actually moving a part of a project over to another phase or task within that project, or to another project entirely.