When can we expect a big demotion?

I don't know if there will be an actual demotion where any 5* exclusives get added to the 4* pool.

I think it's likely, however that some or all of the current 4*-5* units get demoted down to 3*-4*. At least Tailtiu, Soleil, and L'Arachel should since they have been available for a long time now.

We didn't get a demotion for book II, so there's no reason for expecting one for book III.

It did not occur for book II, The only big demotion was a month after the first anniversary I think?.
I like to think I was partially responsible for that after harassing the NoA and official FEH twitters for over a month

We don't expect one, we pray for one