Downgrading SQL Server 2008 R2 Standard to Web Edition
Unfortunately looking at the MS matrix you cannot do this.  You were able to earlier because evaluation versions don't have that lock, but once you go full edition it's locked out

http://msdn.microsoft.com/en-us/library/ms143393%28v=sql.105%29.aspx

You could try an approach taken here but proceed with extreme caution.
http://skreebydba.com/2012/11/16/downgrading-sql-server-enterprise-to-standard-with-scripts/

[edit]:
This URL has the scripts to recreate logins, etc. that Aaron pointed out
Just a headsup though, this doesn't script out jobs.  If you restore MSDB to recreate jobs, make sure to 

UPDATE MSDB..SysJobs 
SET Originating_Server = 'SQLServerName'


It will also work for downgrades to web.