>I want to replace a field within a table. Basically it is a little encrypt/decrypt function that I have written. Now I want to be able to apply it to a particular field within a table for security reasons.

function UpdateField

local loError, llReturn

llReturn = .t.

try
   use myTable in 0 alias ToEncrypt exclusive && Do we really need to use table exclusive?
catch to loError
   llReturn = .f.
endtry

if m.llReturn
   try

       replace all myField with Encrypt(myField) in ToEncrypt
   catch to loError
      llReturn = .f. && At this point some of fields could be encrypted, some not - perhaps it's better to buffer the table and do tableupdate instead
   endtry
endif

return m.llReturn

Here is a quick sample - you may implement better error handling and also handle this using table in buffered mode...