Try:

update vizkeys set date_used=NULL where date_used IS NOT NULL

>Greetings Mark
>>>
>>
>>Try no quotes and no periods arount NULL.
>
>OK, just tried
>
>>>SQLPREPARE(gnhandle,"update vizkeys set date_used=NULL where date_used<>'.NULL.' ","real")
>
>and it complained about string conversion
>
>so I tried
>
>>>SQLPREPARE(gnhandle,"update vizkeys set date_used=NULL where date_used<>.NULL. ","real")
>
>and it complained about a syntax problem near NULL
>
>then I tried
>
>>>SQLPREPARE(gnhandle,"update vizkeys set date_used=NULL where date_used<>NULL ","real")
>
>and it didn't protest. But neither did it replace a single value.
>
>Jeez!