Microsoft does not officially support this, however, you can change the following registry keys to make the Windows 7 Enterprise installer believe it is updating a Windows is Business / Enterprise.

1) Go to start run type “regedit” and hit enter
2) Navigate to HKEY_LOCAL_MACHINE->Software->Microsoft->Windows NT->CurrentEdition
3) Right click on EditionID and change from Ultimate to Enterprise
4) Right click on productname and change from Windows Vista TM Ultimate to Windows Vista Enterprise

Change this:
hklm\software\microsoft\windows nt\currentversion\EditionID = Ultimate
hklm\software\microsoft\windows nt\currentversion\productname= Windows Vista Ultimate

To this (Business or Enterprise):

hklm\software\microsoft\windows nt\currentversion\EditionID = Business
hklm\software\microsoft\windows nt\currentversion\productname= Windows Vista Business