Upgrading Windows Server 2008 to Enterprise Edition

Yes, upgrading from Server 2008 Standard to Server 2008 Enterprise is a supported upgrade path: Windows Server 2008 R2 Upgrade Paths. The applications should not have to be reinstalled but as with any upgrade you should make sure you have a good backup before proceeding.

Yes it should work fine. Boot into your Windows Server 2008 Standard, insert the Windows Server 2008 disk and run it. Input a Windows Server 2008 Enterprise key or choose Windows Server 2008 Enterprise and then you can perform the upgrade installation.

http://technet.microsoft.com/en-us/library/cc755199(WS.10).aspx#BKMK_Supported