Say I have an achievement in my game, "Collect your first thingamajig", and I publish this to the live version of the game without a progress stat. Later on I decide I want to add some new achievements in an update, including one based on the previous achievement: "Collect 10 thingamajigs".
I create a progress stat for this one which is updated in code, but I also edit the original achievement so that it's unlocked through the progress stat rather than an AwardFirstThingamajigAchievement() function in code.
What are the consequences of this? Will a player that already had this unlocked find the achievement has been relocked because the achievement has been edited, technically making it a new one? Will it remain unlocked, but still cause a little popup for the first achievement on the next Thingamajig collected? Or will Steam simply not touch already unlocked achievements?