A lire dans la langue originale
It's hard to catch up, if you flush you'll remove all your nutrients and it's not even sure that the balls will change pH by flushing without the plant suffering from overwatering because since the balls are sold between 4 and 5 pH, you risk drowning them by flushing before getting the right pH. Moreover, in soil, the output pH may have nothing to do with the soil pH.
Maybe by soaking your plants you can manage, I mean if you put 3 cm of balls, you try to leave 3 cm of water in the pot's saucer, water at the right pH so that the balls gradually change their pH, but I'm afraid that it will water the plants from the bottom.
In the end, the best solution might be to do nothing, since the balls are at the bottom of the pot you'll have a hard time buffering them afterwards, the plant will feel this pH difference a bit but it draws what it needs from the soil which should represent 90% of the material in the pot, limiting the impact of the pH of the balls.
I hope I'm not mistaken.