When I tried upgrading from SQL Server 2008 R2 Express to SQL Server 2008 R2 Enterprise, it passed all tests and then it failed at step "Select features", saying the following error:
There are no features selected for upgrade.
And a window error message:
There are validation errors on this page. Click ok to close this dialog box. Review errors at the button of the setup page, then provide valid parameters or click help for more information.
Is it possible to proceed or do I need to backup, uninstall Express and then install fresh Enterprise edition?
Supported version and edition upgrade paths are listed at Version and Edition Upgrades. Your upgrade path is a supported one:
SQL Server 2008 R2 Express SQL Server 2008 R2 Express with Tools
SQL Server 2008 R2 Express with Advanced Services
SQL Server 2008 R2 Workgroup
SQL Server 2008 R2 Standard
SQL Server 2008 R2 Developer
SQL Server 2008 R2 Enterprise
SQL Server 2008 R2 Datacenter
with the following note:
This SQL Server edition can be upgraded to SQL Server 2008 R2 on the 32-bit subsystem (WOW64) of a 64-bit server. Failover Clusters are not supported in the WOW mode. If you currently have SQL Server 2000, or SQL Server 2005 failover clusters on the 32-bit subsystem (WOW64) upgrade of the failover cluster to SQL Server 2008 R2 is not supported.
Is your Express edition a 32-bit install and are you trying to upgrade it to 64-bit Enterprise?