Why not convert it at source? Doing it in the DF is a waste of time(CPU) and memory. Unless of course, you are going to use the string value.

I am guessing you have selected the table / view as your source. Don't. Rather make it a query and select only the columns you want and do any conversions there.

You way, again assuming you don't use the string value, will create a buffer to hold the string (Memory + CPU), convert it (CPU), Copy it into the new buffer[no place conversion] (Memory + CPU). What a waste...