>I am wondering when I should use REPLACE instead of assigning the value to a control (eg. textbox) ? If I use REPLACE, programmaticChange doesn't work. Is it 100% safe to just assign the value to the control if the controlsource properly is set and expect VFP to get the change into the controlsource?

ProgrammaticChange will fire in case you (not VFP) will change Control.Value. Basically, ControlSource is safe.