I know it's a bit different, but the way I moved from formula to cows milk with DS was the gradually alter the proportion of formula to milk - so started with 8oz F/1oz C, then 7oz F/2oz C etc until the formula had been phased out completely. Not sure how that would work with breastmilk, though. Along with that, perhaps you could warm the EBM as much as you usually do and add in the cooler milk so there's a gradual transition there (or maybe that wouldn't work so well!)