not sure why the urge to bare root and root prune...? repotting is super simple if you just yank out the pot/roots and all (in an 'undisturbing' fashion) and put it in a larger or different pot (or in the ground). Then there is no disturbance of the roots and most palms just keep on growing as if nothing ever happened. There does not appear to be any advantage whatsoever in root pruning a palm (unless, perhaps, you want to attempt to bonsai it). Barerooting is usually reserved for shipping purposes to lighten the shipment and/or remove potentially offending native soil). Good luck.

Thank you for taking the time the answer, much appreciated. You are right, no real need to root prune, I will leave it be for now. I only want to keep it in the same pot and to switch to a fast draining mix. the weather around here is quite rainy. For this, I need to completely remove the old soil. My main question mark is about the impact of root disturbance while removing old soil: from your experience, is washingtonia coping well with this kind of manipulation? Many thanks:)