Hi. First of all, I hope you know what you're doing, here.
Going from SQL Server 2000 Standard To SQL Server 2008 Express, might not be a good idea.
I expect that you've done your homework on the limitations of SQL Server 2008 Express.
As far as I know, as long as you're using SQL Server 2008 Express, no servers or CALs licensing is required.
Try to talk to the Microsoft's SQL Server Sales department to get it cleared up.