Originally Posted by
KARFA
It certainly used to be - once you checked in it wouldn't work and you had to call to be unchecked in before doing SDC. However, a number of people have reported recently being able to do SDC even when checked in. I think unless there is a need, I would still recommend postponing checking in if you think you will want to SDC.
You can definitely do a SDC after you have checked in, I do this pretty often. You can do it on the app and will get a new BP to download. The only time I have ever had a problem was when I booked a flight at short notice e.g. <48h but I am not sure if that was actually a function of the timeframe or some other issue with the booking.