We can already do this with item showcases; if you have a level 3 showcase and decide at a later point that you only want to show 1 row of items, you can remove all the items from the bottom rows. However for achievements, this isn't possible. Once you fill a slot on your achievement showcase, it will ALWAYS be full. You can't get rid of it, only replace it (without some inspect element browser tricks at least). So if you fill all 3 rows of your achievement showcase but then decide you only want 1 or 2 rows, you're stuck.