If your experience is anything like mine, it will be a hassle. But you're doing the right thing raising it before the flight.
I had no end of problems getting an offset leaf applied post flight. The offset was requested after the booking was created, which I believe is where the problem lies. The QFF team will say the offset needs to be applied at the time of booking which is utter rubbish.
I spoke to several different people without getting anywhere, they all claimed I'd requested the offset incorrectly, or that I had to do it at the time of booking and it wasn't possible to add an offset after a booking had been created.
I had to insist the call was escalated, and waited for a team leader to call me back a few days later. They said they had listened to the recordings, conceded the advice was incorrect, and agreed to credit the offset leaf retrospectively. This took 2-3 weeks.
When it eventually arrived, it was my 5th leaf. It seems 'manual leaves' don't kick off the reward prompt, so after waiting 21 days, I had to call twice to get this sorted. On the second call they asked for my reward preference which was the SC. The prompt never appeared, but the SC didn't about a week later.
Start to finish this took me close to 6 weeks.