Thanks for extending the understanding here of the impact of changes in leverage. It is indeed a significant factor in risk management for all parties.
In normal risk exposure levels I guess it does not impact too greatly on a trader's open positions, but especially traders with small balances and a lack of understanding of how leverage changes the margin requirements on their existing positions and not just on new ones, may well hold excessively large positions and be at risk of a margin call/close out.
As you say, this is a measure of prudency in the face of anticipated increased volatility and it is therefore essential that a broker also warns its traders of forthcoming changes in order that positions may be adjusted if necessary. I guess most reputable brokers do this but I have heard of incidents where traders have not been told beforehand at all!