diagonals and figure 8s

I am doing much better, finally got the rising trot on time and correct diagonal=now I need more help with timing-when I do the figure 8 -at the center of the 8 is where I sit 2 beats then rise again to change diagonal? Or do I have the timing changed before that point? It really is harder than it looks, for me anyways.
I had 2 =2.5 hour lessons-this weekend one on a ottb then one on Sonny-whoeee I am tired this morning but am making progress!

I've always been told to change diagonal on the centreline.

For years and years I was always taught to change the diagonal as I passed over the centre line. I recently started dressage lessons at a new barn and was taught to change as you came back onto the track after completing your change of rein, and to also change your stick/whip over at this point. She explained that you don't NEED to change diagonal any earlier and it could possibly mess up your nice straight line accross the diagonal. I think it's mostly personal preference (don't know if dressage judges would prefer one over the other, but I should think you're safe so long as you're not on the WRONG doagonal, logically) but the way she explained it does make sense.

I believe it has to do with the discipline

In saddleseat, you always change at the center...for what it's worth ;)

I've always known it as, you change diagonals when you start your new direction/bend so right in the middle. But I'm sure there are different reasons and times when you would do it differently.

Thank you that helps

You might want to check out the book 101 Showmanship and Equitation Patterns by Cherry Hill. It explains so much and has great picture diagrams.