>>You are replacing while a memory variable is either T or F. So, if the memvar is .F., nothing happens. Nothing in the REPLACE is changing the value of the memvar unless you have some UDF in the command. The use of FOR and WHILE in a REPLACE does work,, it is just usually used with a logical test of some field in the record set.
>>
>
>While not myVar would be always true, if myVar = .f. So, why this condition doesn't work? From the logical point of view this condition is evaluated to .t., so it should not affect the replace command.

I do not see the point of using a logical Variable in the SCOPE of a replace. During a replace, this would be a constant value. So if true, the replace would occur for all records unless you have another expression in a FOR/WHEN clause. If false, nothing would happen. So using a logical memvar just gives the REPLACE command something else to evaluate [i.e., degrade performance] that is not really necessary. In short, I do not understand the purpose of using a memvar which would have a constant value during a REPLACE. If you need to execute something based on the value of a MemVar, I would do this:

if m.SomeMemVar
   replace FieldX with m.Val1 ;
      for some_other_expression ;
      while [TableX.]Cust_ID = m.nCust_ID
endif