> I saw no price changes other than rounding up or down to the nearest Euro equivalent price

Dunno about Belgium but what I notice in French supermarkets is that prices aren’t rounded at all. 10k SKUs will have 10k different prices (ish).

Plain frozen pizza? 4,62 EUR.

Same with pepperoni? 4,92 EUR

Domestic 500mL beer? 1,14 EUR

Fancier beer? 1,81 EUR