Microsoft announced a sweeping change to its Zune Pass music service (press release is here). The $15 monthly price tag is unchanged, as is the basic outline of the service, which gives you the right to download any album from the Zune Marketplace (an impressive collection, to be sure) and play it on any of three PCs or three Zune devices. What’s new is 10 free song credits per month, which allow you to turn a subscription-only track into a purchase. At the going rate of 99 cents per track, those 10 credits effectively lower the subscription portion of the service to 5 bucks a month. A 14-day free trial is available.