You can use MS SQL Server specific function CONVERT for a better control over conversion

SELECT
   Convert( char(10), [Interval], 102) AS Dates, 
   Replace(Convert( char(10), [Interval], 108), ':', '.') AS Times
FROM dbo.Detail

or you can modify your code as follow

SELECT DISTINCT TOP 100 PERCENT Member_Id,
	REPLACE( CAST(DATEPART(yyyy, [Interval]) AS char(4))
		+ '.' + CAST(DATEPART(mm, [Interval]) AS char(2))
		+ '.' + CAST(DATEPART(dd, [Interval]) AS char(2)), ' ', '0') AS Dates,
	REPLACE( CAST(DATEPART(hh, [Interval]) AS char(2))
		+ '.' + CAST(DATEPART(mi,[Interval]) AS char(2))
		+ '.' + CAST(DATEPART(ss, [Interval]) AS char(2)), ' ', '0') AS Times
FROM dbo.Detail

>In the table there's a datetime column.
>
>The following syntax converts this to two char columns, and it works fine, and it's fast.
>
>

>SELECT DISTINCT TOP 100 PERCENT Member_Id,
>   RTRIM(CAST(DATEPART(yyyy, [Interval]) AS char))
>   + '.' + RTRIM(CAST(DATEPART(mm, [Interval]) AS char))
>   + '.' + RTRIM(CAST(DATEPART(dd, [Interval]) AS char)) AS Dates,
>   RTRIM(CAST(DATEPART(hh, [Interval]) AS char))
>   + '.' + RTRIM(CAST(DATEPART(mi,[Interval]) AS char))
>   + '.' + RTRIM(CAST(DATEPART(ss, [Interval]) AS char)) AS Times
>FROM dbo.Detail
>