SmallDateTime to DateTime Conversion Equality
Converting Smalldatetime to datetime will not cause any issue other than taking double the space for that column.  Look at the the SQL Server Data Type Conversion Chart.

Smalldatetime uses 4 bytes of space compare to datetime uses 8 bytes. Both are fixed length.

From books online:


  When the conversion is to datetime, the smalldatetime value is copied
  to the datetime value. The fractional seconds are set to 0. The
  following code shows the results of converting a smalldatetime value
  to a datetime value.


DECLARE @smalldatetime smalldatetime = '1955-12-13 12:43:10';  
DECLARE @datetime datetime = @smalldatetime;  

    SELECT @smalldatetime AS '@smalldatetime', @datetime AS 'datetime';  

    --Result  
    --@smalldatetime          datetime  
    ------------------------- -----------------------  
    --1955-12-13 12:43:00     1955-12-13 12:43:00.000  
    --  
    --(1 row(s) affected)][2] 


I know in some blogs and articles it is mentioned that implicit conversion will cause a scan instead of seek or will not use the index at all. Which is true in many cases but not always.  In this case you will get a seek. See this article by Jonathan Kehayias.