Thanks to Edward and Marc for having helped me in the STRTRAN to replace everything no matter what the case is.

Here's the function I built to solve this:

CheckIt('michel','notes')

FUNCTION CheckIt
* expC1 Text to replace
* expC2 Name of the memo field
PARAMETER tcText,tcMemo
LOCAL lcText
LOCAL lnPosition
lnPosition=ATC(tcText,&tcMemo)
DO WHILE lnPosition>0
   lcText=SUBSTR(&tcMemo,lnPosition,LEN(tcText))

   * Replace the memo field temporary with a string you can be sure is not part of it
   REPLACE (tcMemo) WITH STRTRAN(&tcMemo,lcText,'UNIVERSALTHREADONTOP')

   lnPosition=ATC(tcText,&tcMemo)
ENDDO

* Now, lets do the final replace
REPLACE (tcMemo) WITH STRTRAN(&tcMemo,'UNIVERSALTHREADONTOP',tcText)