Alter Time of DateTime Field
First thing is you will need to stop performing shorthand math operations like + against date/time values. This breaks when you switch to the newer types, which you should do.

For this particular problem, you can simply use a CASE expression to either (a) return the datetime as is, or (b) convert to date and then add 11 hours.

CASE WHEN DATEPART(HOUR, col) &lt; 11 
  THEN col
  ELSE DATEADD(HOUR, 11, CONVERT(datetime, CONVERT(date, col)))
END