A lot has changed recently between XM Satellite Radio Inc. and Sirius Satellite Radio Inc. judging by the agreement the companies signed to develop unified standard satellite radios, probably by 2004. Clearly, the agreement reached mid-February to unify satellite radio standards is substantial because it should speed growth by lowering subscription costs. In four years, if automakers choose, drivers may have the option of having XM and Sirius (formerly called CD Radio) broadcasts in their vehicles. Right now, automakers are signing exclusive contracts with XM and Sirius. That means drivers only will be able to receive either XM or Sirius programming, depending on which vehicle make they buy. For instance,Motor Co. currently has a deal with Sirius while XM is paired with Corp.