If you want to reatain the old environment you will need to copy the old database to the new environment -
You actually could use express - I believe it supports up to 10 hosts - you might also want to look at the upgrade guide because it provides information on how to move from sql express to sql server - http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-511-upgrade-guide.pdf