Upgrade from Win2KS to Win2KAS?

One of my clients decided to do clustering on his Distributed Applications Server that we’ve been setting up. We’d rather not start from scratch. Does anyone know if this Windows 2000 server can be upgraded easily to Advanced Server or do we need to start from scratch? Obviously we’re leery to upgrade an operating system but if it can be done with some confidence, it will save the client a lot of money.