SOLVED.
There is no "animation function", or anything similar, since changing the frames of items of a collection view inside your own custom (flow) lay out just can't be animated.
The solution to the problem is writing an algorithm that calculates all the relevant frames that the method layoutAttributesForElementsInRect: provides, so that setting the frame of an item will respectfully set another item's frame an will compensate for the added size, or reduced size.