You most probably will not. We have experience in doing tech stack migrations with a number of our customers and they have never ever suffered any downtime. Admittedly, it sometimes felt as if we were replacing a tire on a car racing at 100 mph but nevertheless we did it (and still can count to 10 on our fingers). This outlook may seem unreal, but it shows how we are entering into a completely new phase in surgical migrations. They key here is knowing what to do when, and how. Hard to explain, no exact science neither. More a craft than anything else, but you’re sure as hell better off leaving it to professionals if you don’t fancy downtime.