Most importantly, you've changed. Playing an instrument regularly adapts your muscle memories most precisely to its individual response. Not playing that particular instrument for a while means that you have to take some dedicated practice hours to remind your face etc. how to play it. A personal example: I play bass trombone regularly; I play large bore tenor trombone irregularly. Every time I have to play large bore tenor, I pull my 88H out of the loft for the task. Doesn't matter if it's winter or summer (making for a 20-30 deg C temperature difference in storage temperature), the same thing's occurred - my face has forgotten what to precisely do with an 88H; it remembers the basic outlines, and the playing results sound acceptable, but it feels unresponsive and uncomfortable until I've put a few hours in on it. To pick a computing metaphor, I need to take the time to compile the code for it to run efficiently; it's running interpretively i.e. inefficiently until I've done that. We don't need to take a position on whether some hypothesised metal effect has occurred in the (short? long?) time the instrument's been put away; you wouldn't be able to tell because of this human effect, which would completely mask any but the most enormous difference (and we certainly aren't seeing anything so obvious). As a (usually) wise man was fond of saying on a trombone forum far away and long ago - "The soft machine is more flexible than the hard machine". If a brass player has asked what has changed in their playing, or what needs to change, the answer is near 100% of the time: "you, the player".