They can rise up the height of the headgasket. Often on really loose ones they will stamp a circle imprint into the head as they hammer up and down. For one to truly drop down they would have to hammer through the lip, which seems unlikely. Maybe, your mechanic is referring to the engines from a d1 or he is talking about the several thousands drop that happens once they are loose and they were not fully seated at the factory before the top of the block was milled.

 

I talked to him about the difference between the DI and DII blocks, he's a rover mechanic and schooled me on it. I can't say anything either way, I thought people have had it happen, he's seen it in person, you have photos that show it can't happen. I don't know what to say haha.