The online music purveyors are almost identical in services rendered, offering music subscriptions, online streaming and download purchases.

But Yahoo Music Unlimited customers won’t be transformed into Rhapsody listeners overnight. Aside from moving Yahoo’s music customer database over to Rhapsody, they also need to figure out how to keep all that subscription music playing.

Subscription music services give their customers all the music they want for a monthly flat fee. However, cancel the subscription and the music stops playing. The trick in the Yahoo-to-Rhapsody conversion will be to keep all those Yahoo subscription songs playing once customers become Rhapsody subscribers.

So expect those changes to occur in the coming months, as Rhapsody settles into its new Yahoo digs.

The news comes on the heels of Microsoft’s hostile offer to buy Yahoo lock, stock and URL links for $44 billion. Microsoft has indicated that such an acquisition might be the only thing that could prevent Google’s march toward universal domination.

Terms of the Yahoo / Rhapsody deal were not disclosed.

“By partnering with Yahoo, we are connecting Rhapsody’s ‘jukebox in the sky’ with one of the biggest music audiences on the Web,” said RealNetworks’ chairman and CEO, Rob Glaser. “Soon, tens of millions of Yahoo users will be able to access their favorite music through Rhapsody – wherever they go, whenever they want it.”