All 100,000 Sprint Spectrum customers in the Washington and Baltimore areas will be moved to the company's newer Sprint PCS service, the nation's third-largest long-distance phone carrier has announced.

While most of Sprint's wireless customers in the area already use the PCS network, the move will affect users of Sprint Spectrum, the Kansas City, Mo., company's first-generation digital wireless model. Sprint Spectrum users in the area will receive a form in the mail from which they can choose new phones, with an array of options. There will be no extra charge.

"You'll have the same phone number and the same pricing plan as before," Sprint chief executive Andrew Sukawaty said yesterday in an interview. Only the handset will change, he said.

The exchange of phones will take place by mail beginning in July. Sukawaty said he expects the Sprint Spectrum network to be phased out by the end of the year.

The Washington and Baltimore areas are the last Sprint markets in the country that have not been moved entirely to the PCS network. The company began offering the service in Washington and Baltimore 15 months ago; Spectrum was launched here in November 1995.

PCS features more modern wireless technology than Sprint Spectrum and generally offers clearer sound quality. It is based on a technology known as Code Division Multiple Access and serves 3.35 million customers nationwide. Spectrum uses a technology known as Global System for Mobile Communications, a technology widely used in Europe.

Sprint officials said yesterday they expect a seamless transition. "We had been running the two systems side by side and we just figured it made more sense to roll everything into one service," Sukawaty said.

He would not say how much it would cost Sprint to phase out the Spectrum system.

"But it will save us money in the long run," he said.

In a separate development yesterday, Sprint PCS announced that it had struck a deal to offer Yahoo Internet content via its mobile phone service. Under terms of the partnership, Sprint's wireless users will be offered a range of services from the Santa Clara, Calif., Internet powerhouse, including e-mail, calender services, up-to-the-minute sports scores, weather and financial news. Sprint said it also has plans to integrate a Web "browser" function into its headsets late this year.

"We're basically giving access to all types of information on smaller screens," Sukawaty said.

Sprint PCS shares closed at $45.18 3/4, up 75 cents. Sprint Corp. shares closed at $111.06 1/4, down $1.68 3/4.