SQL string replace based on WHERE clause

I have a memo field which may contain & and the HTML equivalent &. I need to write a script to ensure that all instances of ampersand are the HTML equivalent. Have the below script but the WHERE clause does not seem to factor the individual instances (strings) of & in the memo field, just the field as a whole... Any ideas on how to accomplish this? Thanks.

 UPDATE 
    STOCKMEM
 SET 
    INETFDESC = CAST(REPLACE(CAST(INETFDESC as NVarchar(MAX)),'&','&amp;') AS NText)
 WHERE 
    INETFDESC LIKE '%&%' 
    AND INETFDESC NOT LIKE '%&amp;%'

Solution

Try this instead:

 UPDATE STOCKMEM
 SET INETFDESC = CAST(
                      REPLACE(
                              REPLACE(
                                      CAST(INETFDESC as NVarchar(MAX))
                              ,'&amp;','&')
                      , '&', ,'&amp;')AS NText)
 WHERE INETFDESC LIKE '%&[^amp;]%'

The first replace will change & to &, and the second will replace all & back to &.

BTW, Please note that NText data type is depricated and you should convert it to nvarchar(max). From MSDN:

IMPORTANT! ntext, text, and image data types will be removed in a future version of SQL Server. Avoid using these data types in new development work, and plan to modify applications that currently use them. Use nvarchar(max), varchar(max), and varbinary(max) instead.