They probably did'nt tell you as it does'nt happen with every watch,but I have had it crop up a couple of times with clients of mine,so I thought a little bit of a heads up before you get it back and it does happen may make you a lot less panicky if it does!
But I tell you what,its just alarmed me a bit that they said they are going to keep your original dial,thats yours bought and paid for with the original sale,if you are buying another dial,you should get your old one back,especially if you have'nt done a deal with your retailer to reduce the cost if they keep your old dial. Whenever I had a dial changed in a clients watch the old dial would come back with it,as it is legally thiers with the original sale,also,when I had the bezel changed in my GMT from blue and red to all black,the other bezel came back too.