I had a bash at explaining it here, albeit in a specific context (£25.00 records) though the premise of the article was to challenge the perceived idea of the term "up front".
https://www.soul-source.co.uk/articles/soul-articles/£2500-and-under-the-challenge-r2929
In a broad sense, I think the notion of up-front culture within the soul scene, and I say culture because it extends beyond genre for me, is the overall approach to the music / venue / aesthetic.
For example, you wouldn't have an upfront night with loads of baggies, and you wouldn't really want to hold it in a damp working mans club. So i'd argue that its part of a shift in the scene on the whole - more city centre based, free entry (doing away with old promotion cartels and the bullying that goes on about 'someones patch'). I wouldn't say its a largely younger movement though...but I od think it has a lot to do with the space thats used!
N.B. Sorry for the typos in the attached link, i'd written the article on the train, on my phone.